Also seperate bellhousings are always for manual trannys. The most common automatic for a slant is a slant 6 904, or it's variants (but NOT from a smallblock V-8 which won't fit) and the bellhousings are an intergal part of the auto's (one piece). There was also a 727 for the slant found mostly in trucks and vans, but I haven't seen one in a long time. There are different hole sizes in the back of the manual bells for different trannys. I don't know those sizes but someone here might.
Yes, getting one for the correct transmission front bearing retainer is very important. You'll also want to get one that fits your flywheel and has the correct clutch linkage mounting bosses or you'll be fabricating linkage from now till Christmas.