Your gas will be about the same. Doing an SI or GSR swap... It really depends on where you want to go with the car.
Or you can Do an LS swap also cheaper then the both mentioned above.. an SI has about 15 to 20 more hp then an LS at the wheels of course depending on engine condition.. But is cheaper. But Like I said, It all depends where you want to go with the car.. Fuck an H22 unless you plan on drag racing the mofo or some shit.. heavy ass motor for that car.. well Ill say about 70 to 90 more lbs then a GSR, but not 100 % sure on that.

Honestly, If you have the Money to do a b16, save some more and get the GSR. =]

and btw, AC will be no problem.