I read an article from the buick performance web page on a 68-72 Skylark 350/TH350 to 455/TH400 conversion. In the article one of the first things discussed is that the trans crossmember is different. Where do you find the right crossmember?
When doing my conversion the crossmember from the TH350 worked perfect, just had to move it back to the rear holes in the frame. My only prob was that I didn't shorten the driveshaft and I am now paying for all that hassleou: now the tranny needs seals and I am still out the cost of shortening... Live and learn I guess