I think it depends on what application the heads are used for...my set of 2 stock never rebuilt 4 barrel plain Jane 429 heads are 7/16 shoulder's where the rocker ball is placed, but the threaded area is 3/8's fine thread, for the adjustable valve train...I've seen Cobra Jet heads with the larger 7/16 fine thread...Probably aftermarket rocker studs have also gone to the 7/16 size as well...These heads really like the new roller valve train with the jam nuts...have put quite a few sets on (3) on different engine combo's...Haven't head any problem with the 3/8 fine thread rocker studs...again remember the rocker ball is on the 7/16 shoulder.
I think I remember, that those early '9er's had the heavy cast iron rockers...not good for hi revs...okay for regular old-man driving...too heavy...install guide plates and a good set of aluminum roller rockers...do your homework...it was one of the reasons Ford went to the non adjustable lighter valve train...too many broken and bent pushrods...with the heavy valve rockers...Had a 71 Torino Super Cobra Jet when I was young... I sure loved that car,...my insurance company loved me for the high premiums...lol
For hi revs...try and locate the larger 7/16 threaded stud, similar, though not the same as Chevy rat motor...it's been a while since I've done those heads...4-5 yrs?...hope this helps...what's your plans on the engine? What's it going into? Cheers!