I took my 3 speed out last winter and installed a 700R4 behind my 250 inline 6. Same gear in rear as you have. I had the same problem with highway speeds as you're having. Now I can run 70mph @ 2200 rpm. The tranny swap and installing a HEI ignition are by far the best things I've done to my truck. It is my daily driver, every day. Bowtie Overdrive has just about everything you'll need from info to parts check out there web-site
www.700r4.com/faq/faq.shtml