The 3 blade MT's will slow you down a tad, maybe two or three knots. The MT's are STC'd for the 337 only, it's really not by engine but they are made for the Cont. 360. Not saying that they won't be the same for another STC but there are no other STC's that MT has that fit Skymaster.
The only choice on engine is the Aviation Enterprises STC for the Cont 550. And I'm not for sure that is a completed STC, I think it is. One good thing about AE mod with the 550 is you can do one engine at a time. But that's the only STC, unless you are thinking field approval. The old Riley STC for the 520's is sold and owned by someone else and they are not doing anything with it.
Even with the bigger engine the cruise speed really a huge increase for the extra fuel you will burn. You have to get real high, like the Super Skyrockets do, to really take advantage of the extra HP without filling the tanks every 3 hours. That Skymaster airframe is only going to go so fast. It's really not very clean.
You need speed, you better look elsewhere. I see these guys around me with their Bonanza's and it make me so mad that they can blow the doors off my push pull and burn 1/3 less fuel. Then there's the guys with the RV-10's and that is one performing machine. Climbs like a maniac and cruises 170 kt burning 12 gals. I'm doing 155 kts burning 20 gals. Oh well a nice Bo or the 10 is $150k - $200k and I can buy a lot of gas for those dollars
You need to just call Don, he not real good on the keyboards. Not sure if Owen is still at the AE shop but call Chris his son.