Adam how come you dont have a 3/4" plug in these heads? I worked on 1000s on GM cars, did tune wrk for those dummies for 15+ years. All GMs use like a r45ts, some truck motor homes use R44ts. Even IROC 350 and vettes used 45 heat range plugs. In the old days we used to see Nova's with the 300HP 350s running 10:5 pistons using 43 heat range plugs and vettes with 42s.
On AC plugs
R= Resistor
S= extended tip
T= 5/8 length taper seat plug type.
44 is one range cooler, 43 one lower yet and so on. A 45 is = to a champion in the old N14Y book, a 43 is is an N12 y, and a 42 would be the same an an N10Y which is what a standard high output 340 used from the factory.
Spark plugs are for some an emotional think. They all work fine, just nee the correct heat range, that said I will admit that most autolites have a harder shell.
I would think you would want the plug out in the flame travel and wnat to avoid carbon etc on the plug threads? I have seen heads modified for short plugs though. Is this the reason you have a short plug?