https://forums.maxperformanceinc.com...d.php?t=734861
Found it. 290-7201 studs, trimmed about 1/8" off the bottom. No drill required, just tap for 1/2-13. I used a steel block as a guide. Details in thread.
I guess it all depends on OP's performance needs.
I've done the helicoil thing before, didn't care as much for it.
47 heads seemed to be super plentiful for a while. 1970 11's are also decent medium compression 350 heads.