The OP needs to ask himself how fast he wants his combat-ish airplane to go. I'd guess it will be a sport model, and would expect the .40FP on suction would drag it to about 85 mph on 10% fuel and .015" lines (for solo flying only!) with some sort of APC prop...a 9-7 or 9-8 should do. Considering that our NW Sport Race Ringmasters and Shoestrings would make 75+ on a Fox .35 Stunt, also on suction (supposed to be stock, but some were and some weren't), it seems reasonable.
I'd wholeheartedly suggest getting a tongue muffler from RSM (they're really light an inexpensive) and that running the .40FP on a uniflow tin tank would work just fine for sport. If there's a desire to fly some sort of combat contest, the airplane isn't a good choice (for Speed Limit Combat) or the engine isn't the right choice (for Graffiti Combat). For sport flying, it'll do fine on uniflow.
Not sure what the "large" stock venturi is, but at combat-ish rpm's, up to a .312" bore will work with a .156" spraybar through it, and quite possibly with the smaller OS spraybar. I ran as big as a .312" on a K&B 4055, always on 10%, usually with an 11x5 Taipan in my Humongus. The engine ran fine, but didn't put out nearly as much suds as the .46LA, and was heavier by at least an ounce or two. I don't know where the OP flies, but we have only one site where mufflers aren't mandatory, and the days are limited on that one site. "Muffler up" and save a site!
Steve