The bellhousing match engine size, BB versus SB, but the real match is with flywheel. The bellhousing matches FW dia so starter works. If you have an 11" FW (143 Teeth) you need the earlier 68-69 bellhousing. If you want to use a 10.5" FW (130Teeth) you use the 70 up Aluminum BH.
Clutch can be varied as some 11" FW have holes patterns for both. The 10.5" FW will only accept the smaller clutch, or the 11" scallopped.
The other thing is matching up Z bar. Brewers web is a good reference for seeing an understanding the parts match.
The bearing retainer on the trans does need to match the BH. Some accepted smaller dia (SB) while later BB had the larger dia retainer. There is an adapter ring sold to match small dia retainer to Large Dia BH. While 2 dia retainers for Non overdrive, there are 4 different ones. Both dia can be internally sized for 308 or 307 bearing.
Since your set up was on SB I believe you have smaller diameter retainer, and probably have to go with an adapter, or find a large ring that is internally sized for 307 bearing. This is harder to finde, as it was an early hemi style if I remember correctly.