howdy and welcome, 53texponcho!
I like your approach in terms of the driveline (389 instead of a "3-fiddy shiva lay")
Clarify what is an "M10" trans? (Not being a smar-tass, I honestly don't know)
Can you weld on the '61 and up motor mount perches from a donor frame to your '55 stub? I guess I would approach it that way.
Not trying to kick you out, but that conversion may get more/better attention in the '61 and back subforum:
http://forums.maxperformanceinc.com/...play.php?f=426
Good luck with it and don't be a stranger.