HotRod226; Welcome aboard; I recommend posting your question in the 48-60 forum, those guys & gal are working on like itmes your requessing, also check the classifieds; some times you'll come across a memeber posting a chassis on the web as well. There's even a forum for flatheads; you'll get your answer thru all those mention forums.
The only advice I semi clue on is that 48-51 6cylinder are the same. But don't quote me on it. I just bleieve if you post in those forums you'll get the right answer.

Mitch