If the Offy's were mine, I'd probably make steel inserts for 3/4" plugs (threaded on the outside 5/8" long) - with 3/4" reach 14mm threads on the inside. I'd modify the heads to bore/tap the original threads such that I could screw in the inserts - screw them in with loctite. I wouldn't worry about stripping the short aluminum threads ever again.
But, I wouldn't do this at all - when I can order other heads that work out of the box - that already use 3/4" plugs (Edelbrock).
