I think the 290-D2 crank is 10# lighter than the 320. Also the 290 sump is a 3" bore carb, the 320 a 4 inch.
Fly cutting a 290 case for 320 cylinders and sumping a 4 inch carb is a cool mod, and adding the breather on the back of the case to stop the 290's front cam sling overboard
pushes a lot of power for the weight to
strength- not your case, pun....
Mahlon?