The 82 GMC I need some help!!!

[Squarebod]

I have an 82 Shortbed been playing around with it for a year now so here it goes.
I took a 350 out of an 82 RV 207 cast block.
Went straight to the machine shop had the block bored .030.
Machinist said I had a desired crankshaft so he polished it up for me.
Also installed new 12cc valve relief dish pistons. Stock rods.
Running a comp Mutha Thumper roller cam pn-12-601-8 to be exact. (Did not degree due to not even knowing about that I am a newbie as most would say but I am loving this project)
Double roller timing chain
Dart 180cc intake 64cc SHP series aluminum heads with .015 gasket and .028 deck clearance from block
Holley strip Dominator single plane
Assualt racing distributor 65k coil.
Holley 750 dp 4160 with conversion block 70pri jet 73sec jets 2.5 power valve
Th350 2500 b&m converter
3:42 gm12 bolt Spartan Locker


Here's my problem truck idles great in park outstanding throttle response then I drop it in gear and it just falls on it's face dies instantly doesn't sputter it's dead soon as it loads it idles about 1000rpm right now. I do have one soft spring and one medium spring in my distributor (a little trick I read don't know how it affects ((newbie))) is that a bad idea what springs should I run with an automatic or does it matter I only make about 6hg of vacuum at idle. Is it my carb do I need different jets or PV. What's a good timing for this setup got it 10 degrees initial and 36 total is timing my mistake?!?!

Please help me out thanks in advance. My 355 thanks you too haha