Kenny
The 3W-50 has the shortest profile of any gasser I've seen in this category. When I chose mine, I downloaded a lot of dimensional drawings and made templates and fitted them on the plans. I did modify the cowl on mine, but I think that with a one inch prop extension the 3w would be a pretty close fit with the stock cowl. Definite firewall mods would need to be made. If you haven't started the kit yet, I'd make the firewall out of 1/4" aircraft ply, and then make a sliding engine box out of 1/8" aircraft ply, and mount the engine to the box. then you cut holes in the firewall and the former behind it, and when you have the engine positioned where you want it, you epoxy the box to the formers, and reinforce with triangle stock.
This kit does build very tail heavy, so any lightening you can do aft of the wing saddle will be very beneficial, and any weight you can move forward will be good too. I moved the control servos one bay forward, so they are right up against the front of the radio compartment. Good luck,
Sam