You might consider putting this change on the back burner/bottom of list until the truck is otherwise completed and you have driven it for a bit, especially since it is a change that can be completely done without issue on a finished vehicle. This recommendation is not based on $ but on the fact that 20 years on you may view the drivability from a new perspective?
There are rear ratio calculators on line that take tire size trans ratio etc and show mph. They were very accurate when I changed my rear end, I have a T5 and 235 in my 46, went to a to a 4.11 (5th is overdrive) and glad I did all the calcs. Am pleased with the result. My cruising comfort zone is 65mph at most, usually 55-60 is just fine, but to each his own.
Chuck
