I clipped this from a web site a long time ago. Not sure where (possibly TCI) but many transmission sites have detailed info on trans sizes.
A - overall length
B - Length from bell housing face (surface that mounts to engine) to tail (A-B=Tail shaft length)
C - Length from bell housing face to trans mount
D - Distance between trans mount bolts
E - Bell housing width
TH350 (bellhousing pattern - Chevrolet, BOP, Fit-All)
Style
A
B
C
D
E
6" tailshaft
27 11/16
21 5/8
20 3/8
3 ¾
19 1/8
9" tailshaft
30 11/16
21 5/8
20 3/8
3 ¾
19 1/8
12" tailshaft
33 27/32
21 5/8
20 3/8
3 ¾
19 1/8
TH400 (bellhousing pattern - Chevrolet, BOP)
Style
A
B
C
D
E
C tailshaft
28 3/8
24 5/16
26 15/16
4 ¼
20
D tailshaft
33 27/32
24 5/16
27 15/16
4 ¼
20
13" tailshaft
37 7/8
24 5/16
27 21/32
1 ¾
20
2004R (bellhousing pattern - BOP, Fit-All)
Style
A
B
C
D
E
All
27 11/16
integral tailhousing
27
3 ¾
19 1/8