You want to measure your bellhousing. Newer style is 6.29" trans flange to engine flange. For many years engines had a single engine mount in the front and used a mount on each side of the trans bell. This is how the many 6 cylinder engines were mounted. Your bell has those mounts.
As more V8s were installed the mounts were switched to 1 mount on the back of the trans and 2 mounts on each side of the engine. You can use that bell as long as the front to back measurement is correct, also the diameter of the rear hole needs match the trans. I think you need 5 1/8" hole.
Squares came with this style bellhousing:
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This will fit a 12" clutch.