I read your post 3 times and I'm thinking you are considering replacing your 2bl with a 4bl to make-up for some possible differences in rear end ratios. I don't believe a 4bl is going to achieve your objective, in fact I'm thinking your are better off with your current carb if the rear ratio is more highway friendly.
What I would suggest is to nail down exactly what the new rear ratio is. Then you will have a starting place of what direction to take to prepare the truck for your intended use.
Please consider this as friendly input and good luck with whatever choice you make.
In terms of answering your question, I couldn't come up with more wisdom than Carbking.
