Sounds like you have a L45 4 speed. The L50, L52 and L52HD 5 speeds should bolt right in, I think that they are very close to the same housing. If I know what I'm talking about these four trannys are all 20.75" long and are interchangable. This would be the easiest and most simple way to get a 5spd. The strongest is the L52HD, (also Marlin can upgrade the L50 to a L52), check out the transmissions page on the marlincrawler website. Look up the L45(yours), L50, L52 and L52HD and do a little comparing. Also check out Marlins profile and look at what he's running.
I just looked at the MC webpage and the years are;
L45 '81-'82 carb,
L50 '81-'82 carb,
L52 '83 carb,
L52HD '81-'83 carb.
Hope this helps a little.
