RA,
Believe it or not the 21's do not dunk the 15" engines as bad as the XB-2003's do. It does look like a V-4 on the back though, maybe an alien cowl 260 would look better.
I have a customer running a 2+2 in Wi. with a 15" Modified 260. He's on a small lake and runs a 24" over hub Yamaha...