You might consider purchasing this kit.
Not only will it tell you what size theaded fastener you’re dealing with, it also repairs said threads if they should become damaged.
Tester
The front brace bolt is an inch-size bolt, and the rear brace bolt is a metric-size bolt? Will wonders never cease! No wonder the OP got confused. Anybody would be confused by that configuration.
GM has been “metrifying” their cars since the late 70’s. That comes with a big “but” however. If the part wasn’t re-tooled, the fastener didn’t change from English to metric. The 350 Chevy maintained English bolts in various spots until it was replaced with the LS series engines. My '04 GM truck still uses English bolts on the rearend cover but then so does my '07 Ford Mustang.
Well the 3/8-16 bolt didn’t fit either, it’s too wide. I almost thought it fit but I ended up just squashing the threads.
I still think the entire truck is metric 
Ugg. Can you just re-tap it to fit?
Did you try a fine pitch version of the 3/8 bolt, 3/8 - 24? If the pitch doesn’t match up, it won’t start threading and can act like it is too big for the hole. When you say 3/8 “, you mean the diameter of the bolt shaft right? Not the head and wrench size. The wrench size for a 3/8” bolt is 9/16 ".
hmmm … 3/8 " is appx 9.5 mm. So the next closest common metric size is 10 mm, an M10 bolt in other words, and if 3/8 is too big, an M10 is even bigger. So that’s no good.
The next smaller metric size is 9 mm. That’s a pretty uncommon size from my experience. 9 mm bolts come in pitches of either 1, 0.75, or 0.5 mm.
M10 is a very common size though.
The next smaller inch size below 3/8 is a 5/16" bolt, which comes in 16 or 24 threads per inch.
The sae bolt won’t fit properly, because the entire truck is probably metric
If the OP is still reading as with almost anything you want there is Amazon.com-- Use the search for finding bolt sizes and they have several things for reasonable prices.
The M8 I can push in, just scrapes the threads. The M10 is too wide. I wouldn’t think the threads could be messed up on both sides the same. I’ll check a junkyard sometime. I’m giving up on it for now. Both mounts are hard to reach, especially driver’s side.
Why not just tap the holes out to a common 10MM X 1.00 or 3/8 NC and be done with it? A bit larger is not going to hurt anything.
I’m not familiar with that particular bolt but using a 9MM shank size is crap. However, it would not surprise me as GM and Ford also use those goofy 15 and 18 MM wrench size bolts. That’s when you appreciate the Japanese and their 8, 10, 12, 14, 17, and 19MM wrench size hardware.
You had mentioned hardware store bolts so be sure that whatever you use is at least Grade 5. Many hardware store bolts are Grade 2; or fence post bolts as I refer to them.
@ok4450
"However, it would not surprise me as GM and Ford also use those goofy 15 and 18 MM wrench size bolts."
“That’s when you appreciate the Japanese and their 8, 10, 12, 14, 17, and 19MM wrench size hardware.”
I started out buying tools and taking things apart and repairing things (bikes and soap box derby cars, etcetera). Everything was SAE.
My first vehicular “backyard mechanic” experience involved my Sears Allstate Puch Moped and 64 VW bug and a little later I worked at a Volkswagen dealer. “Goofy” 13mm and 15mm wrench size fasteners were used and I acquired more tools and eventually they became very familiar to me.
Then I bought my first of many Hondas and Suzukis when Japanese motorcycles hit the U.S. market and I found the “goofy” 12mm and 14mm hardware. It took some getting used to it.
Then later, on my GM cars I found some SAE and metric. I learned to just roll with it. None of it seems goofy anymore.
I’m glad I never owned anything with the goofy Wentworth stuff! That is goofy!
CSA
Yeah, I once has a bike that required Wentworth. Brings back memories.
My Favorite “Goofy” Wrench Size Is 19/32" .
I’m sure many of you know where this goofy size came from. I didn’t know the story on its history so I did a little search.
It seems that using 32nd inch increments for wrenches and fasteners was common practice
(13/32", 15/32", 17/32", 19/32", 21/32", 25/32" 29/32") up to WW2. The government wanted this simplified and went to 1/16" inch.
I have to accept this internet answer until I hear otherwise.
Apparently folks who work on old model-T’s and Model A’s know this. I didn’t.
CSA
I refuse to buy more wrenches. Geeze, I’ve got two sets of SAE and metric sockets and wrenches and that’s enough. No more room. At the can plant, the Plant Manager forbid the mechanics to use adjustable wrenches on the equipment, so they all had big tool chests. If I needed a 19/32, I’d grab the crescent wrench myself. Maybe I’ve already got one. I’ll have to check.
“I still think the entire truck is metric”
I can’t tell if you’re serious or you’re trolling. I can guarantee that vintage GM is a mixture of SAE and metric. My 1999 Monte Carlo is half SAE and half metric. I’m going to make a educated guess that a new Chevy is all metric.
When they first came out with metric in the us auto industry, my dad called those crescent wrenches “Metric Wrenches”.
It wasn’t too long though that he was buying metric sets for his box because he kept finding more and more metric fasteners on the cars.
Yosemite
In looking through the GM parts diagrams for the 4.3L V6 from 1995, it appears that all of the threaded holes in the block and heads are SAE, many being the 3/8-16. This includes bolts for the engine mounts, transmission mounting, exhaust manifold, and water pump and other front accessory mounting. Even the internal bolts, such as main bearing and rod bearing caps, and head bolts are SAE. However, many of the things that are mounted to the block/heads that themselves have threaded holes are metric, like the intake manifold and front accessory bracketry.
The 2.2L 4-cyl. engine is different. It seems to be all metric.
I have a feeling that those brace holes in the OP’s block are 3/8 but are boogered up. That may be why the braces were never replaced after the engine swap.
I’m a professional mechanic, and I’ve been doing this for quite awhile
Most of the vehicles in our fleet are domestic, many of them being GM or Ford
So I definitely know a few things
I assure you I am most definitely NOT trolling
But you can flag me if it’ll make you feel good :trollface: