With a 52 span go for the 40. add the second firewall , add some tri stock between the the firewall and fus, lot of room, chop out the battery tray
add a new tray to hold the tank at the right height and of course brush the front inside area with thinned epoxy to fuel proof, I did their beaver and have an enya 25 diesel in it my KMP beaver will get either the OS32 SX or ST34, I have done some smaller stuff too the hobby lobby little telemaster
has on TT10 diesel conversion great park flyer, just go over all inside seams with epoxy. If you go back a few pages on the diesel site you will sse the beaver conversion martin