If you can get the 4.1o pumpkin, that is a good start. Find an OD transmission, and that will be even better. There are several OD trans that can be made to work. The tremec listed above is probably the most expensive.
I would not reccomend a t5 in a 3/4 ton truck, but that will likely be your cheapest option... again I think it may not live long in a 3/4 ton truck.
The A833 4 speed is another option, a litle more than a t5, but a much more solid transmission. 1st, 2nd, and 3rd match up pretty well to 2nd, 3rd, and 4rth of the sm420 you have. 4rth gear is a 0.73:1 ratio giving you a 27%OD. With this transmission you would lose your granny low, but gain the OD.
The NV4500 is another option a little more pricey than the A833. This is a really tough transmsion. 1st gear is a little bit higher than the first gear of the sm420... 5.61:1 compared to 7.05:1. 2nd, 3rd, and 4rth, are roughly equilivant. 5th is a 27%OD. The 1994-1997 Dodge 2wd NV4500 is the easiest version of this transmission to adapt to a stovebolt because it is the only version that can accomodate a mechanical speedo gear.
There are other transmissions to research, but these will give you plenty of reading. In the end, you have to decide which one fits your needs and budget best.
John
