need 8 1/4 advice

i have an 8 1/4 behind a 360/904... rear tires are junk bias L60s... but i will be running similar size, maybe a bit smaller and a bit wider in the future. im looking to get into some steeper gears in order to make the car more street friendly. wont see a track, and probly wont see highway speeds... rarely maybe. i cant find any information about swaping gears in an 8 1/4, but have found tons on 8 3/4. i found an 8 3/4 on CL complete from drum to drum with 4.10 suregrip. that sounds fun. but id need the axles swapped to bbp to run my centerlines... so id like to keep the 8 1/4 and just put newer gears in it. how big can i run on the 8 1/4 ideally id like to run around a 3.9. im looking for 0-60 speed, dont care about cruising road trips.