1981 should have poured foam, Pink foam would depend on what thickness you want to use but probably 2-3 sheets of 1.5" A 5.0 could be swapped,but I don't think I would call it easy, motor mount stringers will have to be built, due to the fact the V engine mounts on the sides of the block and the 4 cyl mounts on the front. It can be done as both Watermann and Jasoutside have a Chieftain and an Islander respectively