The cheapest easiest bolt up is probably the SM465...they were used in GM pickups, vans, and medium trucks from the late 60's until the mid/late 80's they are nearly bullet-proof. You can pick them up used for $100-300, and get them rebuilt for a few hundred more. However the only thing you gain with it is a granny-low 1st gear...there is no overdrive.
Another 4spd option is the NV883. They were used in Chevy and Mopar trucks, cars and vans in the 70's and 80's. They have an overdrive. However, I'm not sure of their availability, I don't think they were uncommon, but certainly not as common as the sm465.
The NV4500's are great you get the best of all worlds, but the Chevy versions are pretty rare, and can get kind of expensive, but there are a decent amount of guys who have done the swap into their CUCV.