Robert,
I used to own a 1969 Boss 429 Mustang. MY # was KK429 NASCAR 1998.
It is a 429 CID with 4-bolt mains, forged crank, giant oval-shaped intakes, semi (as mine was) or full HEMI heads, and it didn't use head gaskets. The heads were separated from the block by little O-rings.
They were rated at 375 HP but reality was more near 550.
The car was totally awesome and I regret selling it to a man in Portland, Oregon back in 1977. I was getting married and thought I needed the money.
All I have remaining is an intake valve (it's huge), some photos and some super 8 film.
Thanks for your posting and if you ever put one in a MKII (I don't believe it would fit a MKI) be sure to let everyone know.
Thanks a bunch,
John