We could give you much better advice if you posted some pics of your kart...
You can click on this link for helpful info on doing that:-->
http://www.diygokarts.com/vb/showthread.php?t=737
_______
But the best way a tall guy can get more leg room: Is to extend the front bumper, then re-install the pedals somewhere between 6 to 18-inches further forward. Because moving your seat back will be transferring a huge amount of weight from the front tires to the rear. And this will greatly reduce the karts turning ability, especially if your kart has a live axle.
I'm 6-6" tall and ride this kart: Check out how far the pedals are in front of the front spindles. I've still got my seat adjusted almost as far forward as possible for good steering grip.
_________
But If you still want to do a side engine conversion, these pics show how racekart motor mounts/frames are designed. If your frame is too narrow, one trick is to off-set the seat over to the side a bit to make more room for the engine. Adding a second side-rail to your frame similar to this kart would be a good idea too.
These racekart motor-mounts simply clamp-on between the 2 frame rails.
You simply loosen the 2 bolts on the bottom to slide the mount & adjust chain tension.
