Since you will be spending well over $1000 in parts and a shitload of labor, go find a worthy 400 with the correct amount of fins, or find a 700 to do. You'll be stuck with a 4 fin 400 all WOPL'd out and can't keep it's cool. Yeah, you may want to keep it for the rest of your life but somebody will have to sell it after that.
A 400 4 fin ain't worth a squirt. 400's are cheap.