1967 C20 transmission swap

[jollygreen]

I had originally thought it was a rear differential problem, we heard a lot of...well, just bad nosies from the differental when I took off the rear housing plate, so I kind of jumped the gun and bought a '57 rear end, the mechanic assured me that they were the same, HO72 I believe, anyways, I knew everything worked in my originally rear diff. so I just wanted to take out the spider gears of the old one and swap them with my new ones...to my surpirse, they weren't the same at all. But we ended up taking my rear end apart as well and well, we just couldn't find anything wrong with it, so we steam cleaned all the guts, and they looked great! Put some new oil in there and the same noise came back but not as bad at all.

So now I'm thinking it's the transmission, I have the original 3 speed in there but I heard a popular switch is to the 4 speed 700RH transmission. Am I correct about this? I found one online for about 1k and then another one in town for about 500 installed! I'd like to do the work myself but that's almost too good of a deal to pass up.
Any help and hints y'all could give me I'd be grateful!!!!!
Thanks y'all