The rear exhaust engines stack might not clear the top bit of the Hayes mount, but a little work with a Dremel should make it work. I spoke with Mike H. today at the contest. He crashed his "KISS2" yesterday, when the engine quit without warning climbing out of the Clover. He tapped the plastic mount for 4-40 screws, and just screwed 'em in. The corner of one of the mounting lugs was broken off the case at the front hole, but the screws held into the plastic plenty well enough. One screw was bent! Put some LocTite on the screws, snug 'em down and crank them 1/4 turn more. Never check them...just have faith.
A built-up profile with cross grained sheeting...or 1/64" plywood...would be very good, especially if your profile rules will allow a thicker fuselage. Ours allow up to 3/4" max at the TE (based on AMA rules for profile scale or ?). I'd definitely taper it toward the tailpost. I'd make the core 5/8 thick back to the TE, taper one side, skin it, flip it & shim up at the tailpost, plane the taper into the second side, and skin that. Should stay straight, right? CA! Otherwise, make the core (like a Comet stick model), taper both sides and skin both sides at once? I think it would be worth the effort, but I'm not sure what's the best way to do it. I did an F1A fuselage like that, and it came out straight...I think I did sheet both sides at once. Man, that was a long time ago! OTOH, 1/64" ply on both sides of a light 1/2" balsa slab is uber stiff...use slow CA or epoxy.
Steve