Transmission questions

[jnf2k2]

Hey everyone,
The transmission on my 76 K10 is going to probably give out any day now and I'm looking for some help on what to replace it with. It's got a TH350 in it and I'm not too keen on rebuilding it since I'd like to have a 4 speed, maybe a 5 speed. Right now it struggles to get up to 45/50.
So basically what would be the best transmission to go with for what I want? How much should I expect to pay, would it be any easy swap, would the transfer case need to be switched out as well, etc. Also somewhere down the road I'd like to put a 454 in it, it's got a 350 now, and what transmission would be fine for both.
Any help or advice anyone could give is highly appreciated.