I have a C5ae-a 391 FT no bosses.

I've done three 63s, one convertible and two XLs. That frame mount Jim posted worked, all the driveshafts and yokes were the same. Have to change the starter and flex plate. I had to do something to the emergency brake cable but got it to work.

Had this same crack on my block, it went all the way across to the valley. I bottom tapped it as far down as I could then filled the hole with JB weld and found a rubber o-ring to fit tight over the stud and trimmed out the gasket so it just fit inside, torqued it up while it was wet and it worked well with a Felpro gasket for years,just did it again with Cometics but haven't fired it off yet to see if it works.