Generally, if you'll check the owner's manual/warranty/instructions on any bike, you'll find that they don't recommend motorizing it. So when you start looking for the best bike to motorize, it's kind of a challenge.

You say you're 6.5' tall, which means you won't find very many bikes that'll actually fit you. Something like a Walmart cruiser is going to come in a one-size-fits-some size, and that will fit "average" size people. If you're riding it via pedals, probably won't work so well. If you're not pedaling, maybe it'll work better- it would help to sit on one at a store, though.

Weight ratings on a bicycle are VERY approximate, and how things work out will depend on how many miles you put on it, how many bumps you hit, how many curbs you hop, etc. So it's not like a bike rated for 300 lbs is good for 290 but not for 310. Generally, the wheels on a bike are the weak point, not so much the frames. You'll find discussions in the Clydesdale forum about the best (unmotorized) bikes for heavier people. The speed ratings on the motor may be based on a lighter rider as well, so if it's possible performance would suffer as well.

I think I've seen one person riding a gas-engine bike one time around here. I've probably seen 2 dozen or more on the local Craigslist. So it really does seem like they are way more popular to sell than to operate. Maybe I'm must not in the right place at the right time or something. But the noise does seem to be a drawback, and I don't know if that's really what's behind the issue or not. Anyway, the idea seems to really appeal to people, but the reality seems to be a bit disappointing.