Ditto on Steve's comment on the torque tube. If you want a higher-speed rear-end, the 3.55 ring/pinion from Patrick's (as posted by Scott) lowers your RPMs and allows easy cruising up to 65 mph. If you want safe, higher cruising-speeds, you'll need to consider a variety of other changes.

If you want to mate a different engine (from a 216/235/261) to the torque-tube transmission, wait to hear from someone who has actually done this. Maybe Steve has information on how to do this.