I looked at this before.

The only real difference between late 320 and V8 rads is location of in and out. Inline 6 cars had in / out on left side and V8 one left and one right.

The 300 did use a shorter rad and taller mounts. These mounts are welded to the lower bar so some cutting will be involved to fit the 320 in a 300.

If you have 320 parts sitting around, that is the most direct route though a SL320 rad is $ 400 last time I bought one.

Baring 320 parts sitting around, I'd move towards a more common off the shelf rad from another brand as it might cost 150 and you might be able to use the fan / shroud that was made for the rad. Custom stuff is $$$ and can't be found at the local auto parts if you have a failure on the road.