Dave covered most of the questions.... beam mount.. makes service and installation easier. The very original QM40 designs were actually set up to install the Q-500 engine with the backplate mount. That went by the wayside about a year after we started with the class.
For sport use, you can stick just about any "40" you want in the nose. My old 1995 vintage Miss RJ is still flying......has an OS 40FSR up front and its current owner is having a blast. I also did a bit of test and experimenting using my Bob Wallace mustang, with a Thunder Tiger .40 in the nose (prefered 424 engine)... very nice performance.
The more current QM40 designs are geared around the Jett and Nelson engines. You can purchase either with an integral (driven backplate) spinner of the correct size, and both Dub and Dave Shadel can provide you with the correct engine mount. The engines themselves are pretty much plug-n-play. Current APC QM props make life easier too.
As Dave noted.... its probably a good idea to either go with a Q-500 engine, or simply with a SJ-46 or 50..... more user friendly, and you can throttle back if needed. The only issue may be clearance for a 9" dia prop... but I think you will be ok.
Lyle use to sell the Dago and Strega with a foam core wing option....... basic kit format. Most competitive guys ended up buying the composite wing version of the Dago. For sport use, that would be an easy/econimal approach if he still offers cores. My Strega had a foam wing... nothing wrong with that.
You will not 'need' retracts
And its not worth the weight or complexity in this size model. Turns out, unless you button up the wheel wells with flush fitting doors, the Pizza cutter wheels and struts have about the same drag as a turbulent open wheel-well.
If you want all-out speed, and some P-51 looks, go with one of Lyles planes. Danny Kane also made a good P-51 qm kit, Miss Foxy Lady.
Gary.... The Wing AT-6 flys more like a Q-500 than it does an AT-6. The last one I flew had an OS .46 with one of Dubs Turbo-jett mufflers on it. Flew quite nicely..... no bad habits at all..... and was easily over 100 mph. Prop selection is pretty generous on the plane. We tried a few different sizes. Turned out with that combination, we got what appeared to be the best performance turning a 10x7 apc. I did not notice any buffetting or anything of that nature. If I had any complaint, is the plane needed to be about half a pound lighter.
Bob