The questions on bell housings, flywheels and clutches arise here MANY times. So, I took some pictures and will attempt to explain the differences.
First, because it comes up so often, here is a picture of the truck housing and then a picture of a truck housing next to a pass car 3899621 type housing. Notice the big, extra bulge in the lower section of the truck housing. The purpose of this additional bulge in the truck housing is to accomodate a 12in clutch. You don't want or need a 12in clutch in a car.
Next, is the truck housing turned over on top of the 621 housing to show the difference in the bigger hole of the truck housing compared to the hole of a pass car housing.
The smaller 403 type housing compared to the bigger 621 type housing.
The next two pictures are the earlier open bottom housings. There were basically 3 types, 55-57 cast iron, 58-62 cast iron, and the 60-63 alum.
Notice the 55-7 style with the provision for side engine mounts. The 58-62 iron and 60-63 alum housings are virtually identical (no side mounts). The ALUM
open bottom housings were used on hi-perf 348 (pass cars) and 409 engines and ALL 60-62 Vette engines. The 60 alum open bottom is a one year only housing. The difference for the 60 is that it had a smaller hole for the bearing fork than the 61-63 version. ANY of the 60-63 open bottom alum housings are worth a small fortune!!! Also, ALLLLLLLLLLL 57-63 open bottom housings were used ONLY with the big, 14in diameter 168 teeth flywheels.
The next 2 pictures are the big, 14in dia, 168 teeth and the small, 13in (actually about 12 3/4in) dia, 153 teeth flywheels and flex plates. Notice the big flex plate has added weight on one side. This came off of a SB400, which is EXternally balanced and requires a flywheel or flex plate with added weight. The smaller flex plate came from a 307 with PG.
This picture is the engine side of two big flywheels. One is neutral balance, the other is UNbalanced. Notice the added weight on the right flywheel. This is from a 454, but an UNbalanced flywheel with this extra weight is required for BOTH a 454 (and dirivatives such as 468, 496, etc) and a SB400. The 454 flywheels are fairly common to pick up at swap meets, but it is unlikely that you will ever come across a SB400 flywheel. The 454 flywheel can be used when building a SB400 because when you have a SB400 (or any engine) built, you're going to have it balanced. Right? Right! Thus, the 454 flywheel provides the needed weight for a SB400.
The last picture shows the comparison of a 444 and 621 housing. Notice that the ONLY
difference between them is the thickness of that one rib at the corner. The thin rib is the 444 housing.
Last of all, look closely at ALLLLLLLLLLLLLLLL of the housings for the big flywheel. Notice the bulge for the starter nose. That bulge sticks out about 1in from the side of the housing, whereas the starter nose bulge of the 403 style housing is even with the side of the housing.
The smaller flywheels will ONLY accept a 10 1/2in clutch. The big flywheel is required for an 11in clutch. BUUUUUUUUUUUUUT, some of the early BIG flywheels only had a bolt pattern for a 10in clutch, such as 55-7 Chevys with a V8. They all had the big flywheel, but only had a bolt pattern for the 10in clutch (in the early years, clutches were 10in but now they are 10 1/2in and fit the same bolt pattern). There have been some big flywheels with dual bolt patterns for either 10 1/2in or 11in clutches. They are kind of rare.
Study carefully, take notes, there will be a test!