Wow....First things first, sounds like more than 3 hours of work-
Yes, big block engines have been done with launching success (Rossi-65 & Rossi-81DF by Chuck Auger), but I would stay away from them unless it is all you have and you wanna make a 30 second youtube clip. There is plenty of engine mount available to "widen" if you are determined to try that...
Pros of big block: Looks *****in, nobody expects success so if you fail you get credit for trying.
Cons of big block option: Makes an already heavy airframe...heavier. Can be difficult to balance and it swings bigger wood (so to speak) so launch has even more torque. Poor gas mileage.
For servos the 945MG will be fine (they are 100+ oz. @ 6V right?). I have used 645MG's (107oz. @ 4.8, and 133oz. @ 6V)on several Whips and never a problem. If it makes you feel better to buy $100 servos go ahead-
Lastly, if you are going to go to all of the trouble of cowling and fairing the engine just get an Oakdale Phenom. Regardless of what you may have heard there is nothing wrong with them
James