I think I like sixstarved's idea the best, and actually that was what I was planning to do. I already considered the camshaft and head issue regarding the 400. The 455 currently has #15 heads on it (could be either '69 or '70 heads, I haven't checked the date code yet because it's in my friend's garage). Either way, the #15 heads which are recently out of a machine shop would probably be decent on the 400, but I need to buy a cam. What apparently happened with the 455 was that my friend's friend rebuilt it but re-used the stock rods, which were inferior metal. He built it pretty stout and at least one of the rods twisted, possibly breaking the crank as well.
I think either way I'll get the 400 and 400 tranny, and the 455 also. If it ends up I am happy with the 400, I will post the 455 for sale (though I'm almost sure I will want to keep it!)