4 speed od or 5 speed

Obviously you are trying to do this on a budget, therefore a Tremec or Passon, though very nice is not in your future.

You should be able to get the date of manufacture, model, etc from the stampings on the case. Then you could look up the breakdown of the gear ratios. Usually, truck transmission use a pretty steep first gear so you may not need to go to a 4:10 rear gear because of the torque multiplication in first. Let's say it's 3.09:1 in first. Multiply that by 4.10 and you get 12.67. Compare that to a typicalal Mopar 4 speed with a 2.66 first gear and a 3.23 rear end ratio. Multiplied out that's 8.59: coming out of the hole. The early /6 4 speeds used a 3.09 first gear and is a sneaky way to get better acceleration down low while still using a reasonable rear gear. Do some quick calcs and you may find that you can get away with a 3.55, 3.73. or 3.91.

I've come to the conclusion that the biggest gains in gas mileage come from using an OD. I like the budget idea but the trans tunnel (and T-bar crossmember) will definitely need some work. If it's a Dakota transmission used behind a Magnum, you should be able to get it to work using mostly used junkyard parts..