Transmission / rear end set up

[edc]

Hey guys - sorry for what is most likely a duplicate of a duplicate of a duplicate post. I've been searching around on here, but there are SO MANY posts!!! I love reading through the posts, but not when I'm on a mission for something specific.... LOL

'65 LWB
283 V8 :: PS / PB / elect ign
350 auto with towing shift kit :: I think it's a TH?
3:73 rear end :: guessing
Engine and trans rebuilt ~ 3 years / 3,000 miles ago

Last owner used it on the farm sparingly for his horses. I don't tow anything, but I bought the truck to USE it. I haul lumber and furniture and mulch and stuff with it. She ain't a show truck and I have no plans of hot-rodding it.

Problem is I can't drive it on the highway for long at all. It SCREAMS at 70 mph. This is what I need to remedy. (That and I get maybe 8-9 MPG.)

What's my best option?
- Change the rear end? (to what?)
- Go to a 700R4?
- Do both?

Thanks in advance for the input - you guys are the best!