If you were experimenting with a rear wheel drive, solid rear axle car, you could fit it with what is called a 'quick change' rear end. Real common in dirt track racing, a 'quick change' allows the pit crew to change rear end final drive ratios in about 10 minutes. The teams carry several set of inexpensive quick change spur gears in the trailer, to accommodate different sized/configured tracks.
For FE experiments, there is no reason a guy couldn't pick up a used quick change rear end and some sets of gears at a roundy-rounder swap meet for $500. New gear sets are only $70/set retail, there are at least 60, count 'em 60 different ratio available!
Once the fab work of outfitting the car for the new rearend housing, shock mounts, spring perches, whatever, gear changes become a snap.