I'd say got with the GT2 frame and put your own stuff on it... Makes for an easy build and the quality is definitely there.
If you're on a budget, go with a single speed cruiser, of course, if you plan on building a shifter you'd want something with more than one speed, but for the easiest installation, stay single speed.
Also, the bike dosen't have to be brand new and some of the best frames were built in the early 70's and older so hit up some of the flea markets and thrift stores to see if there are any good older cruiser bikes for sale. These frames tend to be stronger and better built but will most likely need to be disassembled, cleaned up and painted, but the end result is a nice strong frame and an excellent bike for the engine to go in as well as saving some money along the way.