Hi all, long time lurker, just joined. I have a couple q's if someone could help me out👍🏻

1. I am putting togather a "mild" 6.2 for my son. Its a original Redblock, with a set of 1407762 heads/D cups. I built a girdle, installed ARP's, also has low compression pistons and a .310 pump.
Heres my issue:
I found one of the valves "broken" on top of the stem. Like the plating/hardness has cracked apart. No biggie i thought. I dug through all of my other extra exhaust valves, probably 20 or so, and the stems are all to big for this heads guide. So i checked, and the original valve was .366", and all my extras are .370". So i went ahead and pulled another exhaust valve and it was also the smaller .366". What gives?

2. Having a heck of a time finding studs for my main girdle. Called ARP and they have AU3.800-2lb, but that will just barely be flush with the nut. They want about $20 apiece for them. I hate to spend that and not be 💯 comfortable with the results. So im checking with another company that may be able to help me out, but they have to see if the studs are over 125,000 tensile.
Any ideas?

3. Lastly, what factory exhaust manifolds were the largest/best performers? I am thinking about a 6.5/4l80e swap into my '05 trailblazer, but the factory exhaust will not work at all. So i am gonna try to do a front/right side turbo mount.

Thanks all