Patrick's has a conversion kit for less than $300 that will work with your current transmission, engine and rear end. You use this kits to change out the gears in the rear end. I have read this is a simple task for a shade tree mechanic. I have not tried it yet. This kit is suppose to get you about 65 MPH.

http://www.patricksantiquecars.com/chevy-gmc-auto-parts.html#chevy7

From the web site
*CRUISE AT 65 MPH WITHOUT ENGINE STRAIN!
*INCREASES SPEED 15% FOR GIVEN RPM!
*FITS STOCK DRIVELINE
*OPTIONAL INSTALLATION KIT (SHOWN) ALLOWS COMPLETE RESTORATION OF REAREND

Dave from Northern Kentucky
My 54 3100