RE: MGT Engine
Most big block engines will bolt up to the stock engine mount without any trouble, but you will require a longer throttle linkage.
You wouldn't have to change the stock muffler, but to really get the most out of an aftermarket engine (as well as the stock one) you need a decent tuned pipe such as the LST1 or 2 pipe.
I suggest you get a better pipe first, and install the FOC if you don't need reverse. These mods will give you better acceleration and top speed, and are much cheaper than a new engine.