I ask about the C-6 because I have one. And I'm not too sure about the belhousing. For instance, a T-5, AOD, (and AOD-E) and the later model electronic version of the AOD-E (can't remember the alpha-numeric designation) bolt up the same on the 3.8 and the 5.0. I'm pretty sure that the 5.0 and the 351 windsor have the same bolt pattern for the bell housing, but not positive. I do know that there are many parts that are interchangeable between the 351W and the 302 (302 HO for the proper firing order). What isn't are the lower intake manifolds (for FI or the intake manifold for carbs) the crank and rods. The pistons, heads, cam and virtually anything else will bolt right up.
I know that most C-6 trannys were put on big blocks, but there were a few that were put on small blocks, specifically in the F-150 with the 351W. This is the tranny that I have. I don't think that there will be too much in the way of mods to the tranny that I will have to other than I have mentioned before. I'm just hoping that some one who has some fairly extensive experience with the C-6 trannys can confirm or deny my thought pattern. I'm looking for more concrete assurances rather than speculation.
As for the cost, well, by rebuilding the C-6 myself and working with the adapter for the shifter as well, I can save a significant amount of $$. I'm sure that there may be a few hidden expenses, but on the whole, relatively inexpensive (less than $1k estimated I think on the high side) as opposed to the AOD which could cost upwards of $2k just for the parts to rebuild it. I'm eventually going to put the 351W and the C-6 in a MN-12 bird. In the meantime, I'd consider this R&D.