High RPM on highway

[Ultanium]

Been working on my 69 C20 Panel truck all week, and have made lots of progress, to the point where I can drive it pretty much anywhere. The PO had put the two piece driveshaft in wrong, leaving the carrier to handle the flex of the rear axle. The truck has a 350 crate engine, and 700R4. Once we got the engine tuned up, and took it out for a test drive, the 350 was spinning at 3400 RPM at 60 MPH. I thought for sure that the 700R4 wasn't going into 4th, but it was. The truck has a Dana rear, and we are guessing 4.56 gears. I hate to lose the Dana, but I think I need to swap something a little more highway friendly, if I'm going to use this truck for cruising. I am thinking a 3.73 gear will bring it down, and maybe a 10 bolt or 12 bolt, with posi wIll be just fine. Will only be towing a light trailer with this panel, nothing heavy. What rears will direct swap without too much hassle? Is the Dana worth anything?

Tom