Sounds like a nice find. Sportsman II heads came in either a 64 or a 72 chamber (most that I have measured are bigger than advertised). I doubt they could even go to 54 with angle milling. How do you know they are 54? Best thing to do would be to remove a head so you can see exactly what you have. Do the pistons have 2 or 4 valve reliefs? I would also remove the cam so you can hopefully get a part number off it and seee what you have there as well. If the headers are 2" sell them and get some
1 5/8" or 1 3/4", 2" is way to big, it will be way down on tq.
The air-gap would be a great intake for this motor, depending on how much cam and compression you have it should be just over 400 hp.