GM is probably different than ford but the garage that replaced my rearend only took a little over an hour, parts were $35 labor was $100
If you have all the tools and maybe a friend it shouldn't take real long, I would guess much easier if you can lift your hole rearend up.
I ended up with 2.8's because that was what was available from his junkpile cheap, great mileage, similar power oddly enough (but I have an auto). These old non-turbo diesels have most of their power near idle so you should be OK.
You will need to figure out what your rear diff is and attempt to move up to a diff with high enough gears to make your 3rd similar to an OD gearing. You may find you can still pull from 2nd and 1st is still enough to pull the hills 20-25% higher gearing change isn't much to the low end gears but really shows on the top gears.
You could do more than that to make your gears into real highway gears but then getting 1st gear right (you would need to know its reduction) would be critical so you could take off without riding the clutch much.
So I guess it depends on how extreme you want to go, my 2.8's were overkill but something more reasonable 3.02's might work OK still.
What is your rearend reduction ratio now? 4.11's?
Cheers
Ryan