I put a TH200-4R behind a 248 in a GMC. Should be basically the same as what you are doing. I cut a notch out of the rear crossmember and left it in place for strength. Pictures here. I also had to cut the floor.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.