I'm using a step-through frame because it's all I have. It probably would work better with a rack-mounted tank but I just have the tank higher up on the top tube nearer to the stem of the bike. I've had no issues since the bottom of the tank is still just barely above the carburetor so it works out. Some considerations are that you will likely have abysmal clearance between the carb and the seat tube, your choices of aftermarket exhaust will likely be limited since there are two tubes in the way instead of one, and the chain tensioner will be the biggest pain you've ever experienced (At least for my bike it was. Since the engine was higher up the chain would rub against the seat-stay of the bike so I had to do some upside-down trickery with the stock tensioner which didn't work for very long at all really since the top length of the chain gets more load than the bottom. I'd get an arc chain tensioner if you're going this route). On the other hand, you have plenty of clearance for any cylinder head modifications. Stuff to consider...
