Cheapest easiest way is likely to rebuild the 3 speed and get the 3.55 gear set for your rearend.
Other options include changing the rear and using a wide range of other transmissions.
Some transmissions that have been used include, but are not limited to the following.
T5 5 speed. (search the tech tips)
A833 4 speed
t10 4 speed
saginaw 4 speed
sm420 4 speed
sm465 4 speed
3 speed with OD
NV4500 5 speed
As for automatics
th350
th400
700r4
200r4
powerglide
dual-range hydra-matic
What engine do you have in your 54? Do you plan on swapping engines?
The Manual transmissions are going to be a much simpler swap than the automatic transmissions.
