If you look on the casting pad located at the front of the passenger side head and just in front of the valve cover, you will see some numbers and letter. There should be 2-3 letters together (example CNN) along with a bunch of numbers including serial number. These letters tell you more about your engine. In my case CNN is a 1970 Camaro, Nova 350 rated at 250hp with auto tranny. I have a book with the codes if you find it. Looks as if Tom has even more details.
. There is also a casting number at the very back of the engine near the tranny on top which gives you a more general identification.
I think the engine is the main difference. I'm not sure if the exhaust was different on the 75.
Mine is not stock so I dont keep up with all that. Unless your trying to keep yours bone stock I would make some engine upgrades to improve the performance. The 75's had the wimp engine of all time, however that is fairly easy to correct with upgrades.
KenSny's right. As far as engine goes. The L82 has a bit more aggressive cam and higher compression ratio ... hence more oomph for L82. The L82 MAY have an aluminum intake while the L48 MAY have iron.