I'm not particularly recommending a cruiser frame BTW, it just happens to be the bike I've the shiftkit on. I like mountain bikes but I had a heck of a time fitting just an engine in mine, it's somewhat difficult to find a new mountain bike with a "traditional" style frame - aluminum & unusually shaped D tubes are the norm these days.
It's also not uncommon to end up replacing/upgrading just about everything on the bike - double wall rims, brakes, crankset (included w/shiftkit), seat, handlebars, forks/shocks... the list is pretty short and it's often cheaper to upgrade than to buy it that way. If you see a sweet older style mountain bike like the GT LTS frame from the late 90's (full suspension) don't discount it as a possibility if the rims are tweaked, you'd prolly want better than it had on it anyway
Check out Ghost0's sweet build if ya want;