Got the jets installed in my FT500 last night and (thanks to J's recommendation) didn't even need to remove the carb. I also decided not to replace the float bowl gasket and thankfully it doesn't leak after reassembly and testing.
Maybe I should've ridden the bike more than once before I changed to the recommended 2x oversize jets, but I'm not looking back. The bike seems to start, come off choke, idle, and just plain run better throughout the rev-range. Took a test spin tonight and although it doesn't have as much brute power as my lightly modified '80 "brand Y" XT500 (Mikuni 34, K&N air filter, Super Trapp, & oversize exhaust pipe), my FT's much smoother and certainly more civilized. Probably won't be modifying the air box or any other visible stuff since this low-mileage gem is such a clean bone-stock example of a first-year FT500. Thanks everyone for being here!