A 75 Malibu with a 383 stroker doesn’t seem to fit with someone unfamiliar with automobiles and with shallow pockets. Maybe someone has a Chevelle they will swap for that muscle car.
That’s a good question for the OP. Did this happen immediately after the distributor cap change? Or did it run properly for some time and then this problem cropped up?
If this thing turned over with the starter just fine prior and without ignition in the picture…and sounds terrible trying to turn over with ignition engaged…AND after a cap rotor and plug wire removal.
I would say the chances of a wrong firing order are extremely high. I would even put money on it if I could.
I also agree @“Rod Knox” …that hardware doesn’t suggest low dollar…but anything is possible.
Fix the firing order before you mess with any starter shims… I can almost see an incorrect order from here…LOL
Blackbird
Shims are static, they don’t change relative to load. It’s not the shims. It’s most likely either something along the lines you mentioned or the juice required to operate the ignition is loading down the supply bad enough that the starter isn’t spinning fast enough to fully deploy the bendix.
…like I said… I got 20 on my theory… 
Hope Im right…it will be a quick n easy fix for the OP.
Ive seen n heard this condition so many times…those uninitiated folk whom Ive helped through it All couldn’t understand how I could know what the problem was prior to any fussing or cussing. I just listened and processed the info they gave me about what they had just done and the symptoms they were having. If your engine does not turn over with the proper sounds After you just had the cap rotor n wires off… The very first suspect is the firing order…I dont know if its history or experience or both that leads me straight to that possibility.
Those who do this enough will be able to tell this condition as soon as they hear the engine get spun by the starter…its quite distinct
Screwing up the firing order is more common than…than…what? I dunno…but its common. lol I think an ez fix lies ahead and it has nada to do with shimming the starter…tho that isnt out of possibility…but I would look to that after I ensured the firing order was 100% correct…there can be no deviation in this…
Blackbird
After you just had the cap rotor n wires off
Well, the OP didn’t say that exactly. He mentioned having issues with it but not that these were immediate subsequent events. I’ll give you credit for making that connection. I’ll usually default to expecting someone to make those event sequences obvious like- now what? I just got done swapping out my dist cap and the starter is acting up! What gives? Then again, been burned by that assumption before 
@“Honda Blackbird” ,
I got 20 that he’ll put #1 at TDC on the exhaust stroke, and have the whole thing 180* off. Maybe that’s what he already did. 
Well I have seen experienced mechanics switch the wires on cylinders 5 & 7 on Chevy small blocks and then spend a week chasing down vacuum leaks, bad caps and rotors, wires, etc ad nauseum. I believe I could trace the scope pattern for crossing those wires from memory and my memory is fading. It’s just so easy to ignore the basics to chase down the unlikely.
I Hear you @TwinTurbo … He did say…he did something with the gas pedal…then the distributor cap…then it seemed like problems with the starter…in that order.
That is what has me thinking firing order. How many times have we seen someone screw up the firing order and then say my starter sounds terrible ? Perhaps I’m confusing other people with ME in my youth! LOL
Im not trying to be a know it all…but Ive made the same mistake when I was “Greener” with this stuff and in over my head as usual… lol Hey the best way to learn sometimes is to have free reign to mess up…it incalculable what Ive learned in the “Screw Up Lab” of life…and Im a charter member…or used to be long ago.
@PvtPublic Aw man…dont go there brother~! LOL… How much typing would it take to remedy that? Oh the confusion! Hahaha
@“Rod Knox” is also right…its so easy to mess up the order when you are distracted…and sometimes…even when you pay attention.
I think we have sufficient clues to suspect the firing order… Its almost text book
I hope thats it… simple to check… in fact its taking longer to write this than it would to ensure the order is correct.
Blackbird
But on second thought I believe it was the wires on cylinders #3 & 5 that were so often crossed. The wires on the drivers side were routed under the exhaust manifold and held by several clips and the center plugs were within a few inches of each other in the exhaust manifold valley. The wire were a real pain to replace.
Andit does sound like textbook firing order/timing problem @Blackbird. Sometimes people cause more problems than they started with when they dive in over their heads.
I hear ya @“Rod Knox” and since I already put money on the firing order being the issue… I have none left over to put more money on Us never hearing what the issue was… Because it took less time to fix than it does to write about it… LOL
Oh the Joys of helping strangers over the Internet… No love I tell ya…No love… 
Blackbird
But occasionally a little sympathy @Blackbird.
Alright guys so the starter needs to be shimmed prob is the mechanic place I took it to is a bunch of rip offs there is a small crack in the engine block on the first two to three treads where the starter screws up into block
So…just clean up the threads with the proper Tap… Very easy to do…
Did you check you plug wires at the distributor ? This would be the firing order. It is something you really should do…also an easy job…just be sure it is correct. You did replace the distributor cap I believe…which opens the door to a plug wire mixup…
Just set the engine to TDC…and then go over each wire…and where it is located on the distributor cap…Do each one at a time…till you verified they are all in the right place. Easy to check…easy to mix up the order when you mess w the distrib cap…
IMHO this is extremely relevant with your problem description…
Blackbird