I just bought a piece of 1 1/4" plate and took it to a machine shop. They made the adapter plate in less than two hours on a lathe and a drill press. They are pretty simple to make if your machinist has a little bit of experience.

Here's a link to the thread that shows mine NV4500 into 1963 c10

This is the adapter plate that was mentioned NV4500 adapter plate

You can use that adapter plate without any machining if you get you an early sm465 bellhousing. As previously stated, sm420's used a 4 11/16" bore bellhousing. The sm465 used a 5 1/8" bore bellhousing. The early sm465 bellhounsings were cast iron and had the angled mounts just like our sm420 bells, but they had that 5 1/8" bore that you need.

If you pick up one of those earlier sm465 bellhousings and swap it in, everything will be bolt in.

Liquidated my projects
Now looking for a decent '47-'55.1 3100, 3600, or 3800