1985 P30 SouthWind New motor engine/trans Options

I just purchased a 1985 P30 27ft southwind that was in storage inside for 7 years. It only has 50k miles on it and its in great shape other than the obvious. The drive Train seems a little tired. The Motor has blown Exhaust manifolds, and I drove it for 10 miles and I suspect a major transmission blow out(fluid pouring from front of trans).

I am a car guy, built hot rods motorcycles and dont mind a project. Given the out of date nature of this 454 chevy/ turbo 400 trans set up. I wanted to post to get thoughts on optoins for engine trans. Given east of drop in and limited re engineering. I am thinking of just getting a fresh rebuilt 454 or 502 and going with a 4l80E auto trans or a 5 speed manual Nv5600 or a eaton fuller 6 speed box. I realize this will require some fab work either way. I like the idea of a manual, both these boxes should be able to handle the weight. Plus no issues of TPS switch and electronics for the auto. I also plan to go with TBI system and ditch the Q-jet carb. Witht the right motor should get 8-10MPG.

I understand that you can get this engine out of the front by removing the grill and radiator cross member. I think you can get it out from inside if your remove the exhaust. I have plenty of time, in no rush so front removal seems the best option.

Any thoughts on this project are welcome