1971 C10 Starter/Timing/Transmission Issue

[GreyGhost85]

Good Afternoon All.

I'm kind of at a point with my restoration where it is almost road worthy and would appreciate any insight anyone has to give. This may be long winded but I try to do as much research as possible before asking questions.
1971 C10 with 1985 305 and TH350C, no major modifications. Once started idles great.

Problems:
1. SBC 305/ TH350C starts here an there. I can start it after sitting for a couple of hours but I can never start it right after it has been running for 20 minutes or so. I have heard of heatsoak from exhaust and placed heatshielding around the starter. I can start it while jumping it from my other vehicle no problems. I have changed all battery cables, cleaned all grounds, replaced ground strap, added an additional ground strap, used some jumper cable to connect the ground of battery to starter bolt. One time i hit the starter with a hammer and it fired up lol and then 20 secs later, no start. When I describe no start it is like, the engine turns over very slow. Sometimes it will start slow then go fast and fire right up. Sometimes it will turn slow and then bam start up. Most of the time, it will start slow and then die all the way down to clicks or short turns. When I jump it, bam turns like it never has before and starts right up. Volts read 11.9-12V and 13.89 when it is running. Battery is brand new, 600 CCA at 0 degrees and 750 CCA at 32.

2. TH350C Stalls in reverse. Had to rebuild the transmission last week because it had 0 gears. Fluid is filled to correct level. During rebuild I noticed vacuum modulator valve was taken out or missing so i replaced it and put a new one in during the rebuild. I added the Transgo 1-2 shift kit, all check balls are in place, all valve body bolts torqued to spec, the whole 9. I have attached pictures of my vacuum set up. 1406 Carb, vacuum advanced port connected to dizzy, pcv valve connected, trans valve on carb plugged. Intake manifold vacuum port to th350c modulator. Th350C lockup plug disconnected. Kickdown cable plugged with delete kit. When truck is on stands, all gears function properly. When on ground, D, D1, D2 all function, the engine drops rpms and i can drive. When put in reverse, the engine rpms drop and then stalls. Torque converter is stock and new. I have checked for vacuum leaks and there do not appear to be any.

The only things not replaced are the distributor and the starter (which was sold over as just replaced upon purchase of my project). Could Timing be an issue here? I read that timing could cause slow cranking and problems when put in reverse because of higher engine load required to go backwards? Like if i don't brake while putting it in reverse it will literally squeak tires and try to zoom. With the vacuum advance disconnected shouldn't the engine idle properly? It idles super high with it disconnected and reverse stalls still.

The truck idles like a champ and just purs smoothly, is there a possibility that not enough fuel flow is getting there for the reverse gear causing it to stall. I swear it acts like it wants to start backing up and then it just stalls. I can feel the load of the engine shift and engage but then it just dies.

Attached is a pic of the harmonic balancer, is the little line grove the factory TDC mark? Or do i have to manually find TDC and mark it.

Again sorry for the long winded details, I try to do as much research as possible before asking. I appreciate any insight.