.370″ tip wouldn’t be more flexible and I would not ream the heads for .370″ unless you have done it before and have the proper set up. I don’t care how proficient you may be in using a drill press on other things, but reaming out a hosel on an iron takes experience, in my opinion. You can ruin a head fast. Just saying this because you said you were new to club building. Why would you ream out to .370″ if you have .355″ taper tip shafts? I might be missing something. If the Project X shafts are .370″, they are not going to fit and you could ream them out, but with great care as I have indicated. No need to ream the irons out if you have .355″ tip shafts.
Again, just take the shafts you are installing (assuming the tip diameter is the proper one to fit the Pings), and dry fit them in the heads to see if they give you enough length to make them the 3/4″ over standard.
Britt Lindsey
I hit the ball very high. 65yr old, 6’4″, 37″ wrist to floor.
I play Ping 5-UW K15 +.5″, 2.5up, 7 iron goes 145yds.
Removing Ping steel CFS Regular, midsize grip and installing TT Project X 115g R5.5 +.75 with Super Stroke Crossline 2.0 Club Grips Jumbo .600 81g 4plus 1.17″ grip
Recommendations? I can drill to .370 if that would be more flexible, I have a reamer.