Here's a handy link:
http://www.grimmjeeper.com/gears.html
Put in nv3550 wide ratio, dana 20 2.46 conversion 3.50 gears and your tire size and it tells you crawl ratio, crawl speeds, and highway speeds
At 65mph in 5th, you'll be turning 1,700 rpms, which is too low unless you live in a really flat place.
With your planned 35" tires it get worse at 1,600 rpms at 65. You won't be able to fight a mild headwind at those rpms unless you've got a big block in the bronco that makes all it's torque right off idle.
An overdrive transmission doesn't make sense unless you regear the axles to take advantage of it and match your tire size. The nv3550's 4:1 first only gives you a 35:1 crawl ratio with the gears you have, so you won't have much added low speed capability and you won't be able to use overdrive.
A np435 would make more sense if you're not regearing. It's 6.7:1 first gets you a 58:1 crawl ratio which is reasonable for a mild wheeler and you don't need the overdrive. You start in 2n'd, which has a bit better reduction than the 1st in your 3 speed so getting it moving off the line will be easier. You'll also be turning 2,300 rpms at 65mph (2,200 with the 35" tires), which is a more tolerable cruise rpm.
What it boils down to is you need to either match your transmission to your axle gears, or you need to regear your axles to match the transmission you want to put in.