Alternator, Starter and Battery are good, truck wont crank

08 Suburban Z71 5.3 1500
Truck was having issues with power shortages and issues with misfiring. Put Reman engine in, changed ignition switch, new battery, alternator, sparkplugs, starter, new sensors all the way around, bought new starter relay juat have not put in yet. Prior to putting in Reman, truck spontaneously stalled out when put in reverse. Was at stop light, truck stalled out and wouldn’t start back up. Fast forward to after reman instillation. Went to start my truck and i couldn’t hear anything past the fuse box. Started the truck for couple seconds from starter and heard a noise that sounded like it wasn’t getting enough fuel (which was why they hurried and shut it off) after about 2 hrs of setting we went back to try and start it again. Only to find out that the battery bought today was drained down to 15%. We can not figure out why my truck wont crank and what is draining the battery. Any suggestions would be GREATLY appreciated.

its possible that the new battery you bought was just bad. have it checked out. also check to see if you have any lights that are staying on. a faulty brake light switch for example will make the brake lights stay on and drain your battery. also make sure all your ground are connected, clean and tight.

The starter may not be engaging because of a faulty neutral safety switch. Charge battery, shift into neutral, and see what happens.

The battery drain could be an excessive parasitic draw. There’s a procedure for checking that.

You refer to “we” and “they”. Is this a DIY project amongst friends or is a shop involved?

I have no idea what not enough fuel sounds like unless you mean cranking followed by sputtering. One would hope this “hurry and shut it off” by whomever is not related to a locked up engine.
There’s a lot of unknowns here.

I did do some research and will me checking the safety switch tomorrow. “We” and “they” would be pertaining to a couple of friends that know a bit about stuff, but were all kind of stuck right now.
I do know about the lights because i have checked the break switch on it before so I’ve checked all lights to make sure none were left on. I’m not sure what they had meant by “it didn’t sound like it was getting enough gas” due to me not being home when they had started it. I had thought maybe because it’s a new reman engine in it that it might make a sound or two working everything through the engine. I did go and get yet another battery today, but decided before myself or anyone else messed with it further that it may be a good idea to see what others may think it is.