Taking '70 on a road trip - should I swap trannys?

[White70C20]

I am moving to central Kansas this summer. It is about 850 miles round trip back to Iowa via interstate. My 1970 C20 has a 350/TH350/4:10 setup. It runs pretty high RPMs on the highway, let alone on the interstate. It gets kinda loud in the cab, of course the straight pipes don't help the situation much (haha). My question is: In an attept to keep RPMs lower, I have access to a rebuilt 700R4 overdrive tranny. My mechanic says it would cost $1500 total to purchase/install it. Are there other, cheaper ways to keep the RPMs lower?

I love the sound of my straight pipes, so I'd really rather not get rid of them. Besides, it was fairly loud inside even with mufflers on it.