I have a VERY leaky transmission on my '48 Chevy Fleetmaster. It is the bone stock and original 3-on the tree trans. It operates perfectly, but it leaks all of the gear lube out within a couple of days. How hard is it to replace the seals in the transmission? Is it something that I could do myself or is it best left to the pros? Does anybody sell a complete seal set for these transmissions? Thanks for you efforts to answer me!