What I always recommend is using front suspension forks and a suspension seat post. This'll greatly improve how comfortable the bike will ride. If you only plan to ride on pavement then get 2" wide road slicks
Agree with the above. Stiff chromoly and aluminum frames don't ride as well as good old, inexpensive tensile steel, IMHO. Tensile steel also absorbs engine vibration well. The steering in a beach cruiser is a bit slower, making it easier to control at 20+ mph.
I stumbled across this video from Bike Berry yesterday that you might find useful. It explains how to cut out cardboard templates to see if a motor will fit into any given frame. Like Cruisers, Mountain bikes also have a strong steel frame but on some of them the triangle is just too small. A simple piece of cardboard should tell the tale.
mountain bike all the way. trust me. you gonna want to take this bike out on the trails, jumping things ( pull clutch in for easier ans safer for the motor landings ), running things over, doing the wheelies like a mad man, get the mountain bike. nothing says sweet deal when you are flying down the road at 35mph and do a manual transfer to a trail by drop off right next to the mini van full of kids and a hot soccer mom. i haven't had as much fun on a bike since i was a kid. i have a lot of money wrapped into my bike shop basement ( personal shop, lol) in parts and motors, and gonna do more. once you get into this hobby, it grows like a fungus. slowly taking over. be careful new bike guy, before you know it, you'll have 5 engines sitting on on the build shelves in various forms of coming to life. you'll have jets scattered as far as the eye can see, you'll have NT, RT, CNS, SPEED, and crazy off brand carbs mingling in the file cabinet, you'll have links of chain scattered like peanuts at real western bars, you'll be an addict for making different engines sing a different song. be very careful, and get a mountain bike.