The biggest downside to the Manafre engine that I can see, besides its initial cost, is the fact that it requires premium fuel. In terms of gas costs that's like taking a vehicle that gets 15 miles to the gallon and reducing its mileage to 12. And the last time I checked Manafre didn't even guarantee any particular horsepower rating for the engine. I think there are more effective ways of increasing horsepower.
I just put one in my '74 FJ40 in March. Have about 500 miles on it so far.
I have more power than before, and the vehicle is a lot more driveable than with the stock CA smog gear. That probably comes more from the headers and the GM TBI I put on at the same time, than from modifications done internally to the motor.
I'm really happy with my setup, but I really can't recommend it for someone else, because it is expensive, and you may well have cheaper options. I didn't want to try rebuilding an engine along with the rest of the project, every single bit of which was the first time I did ANYTHING like it. So the cheap rebuild wasn't possible, and no body works cheap in San Jose CA, so I ordered one shipped on a pallet.
I've only had it off-road about 4 times since the new motor, so my impressions are still forming. It has way more power with the revs up, but I'm not sure if it's got less around 600RPM, or not, than before. The TBI gives great throttle response, but the TBI allows stalling at low RPM's easier than the carb used to. Maybe I'm quicker ion the throttle than the computer is?
If you have more questions I'll try and answer them for you.
Put a Smallblock in it dude. I've driven mine with a v8 conversion for 2 years and can't even imagine going back. Whichever engine you decide on don't get sucked in to the "high performance" craze. That is for drag racing. My first v8 came from a hot-rod camaro and the only time it was fun to drive was on sand dunes. The worn out stock 6 was better. I promptly swapped it out for a mild tow truck engine (a friend found it for me) that runs fine on even the worst fuel. It has the ability to stay lit at what I consider an amazingly low 250 RPM, 400 RPM on steeper climbs. Someday I'll find out what's inside because this one is worth duplicating. The thing to remember is this, TORQUE man, TORQUE. It's a 4x4 not a camaro.