There are two ways to do this. If you have the older TH400 3 speed transmission mated to the GM motor. A 4L80E trans can be put in most rigs. A good built one should cost about $3k. This will also require an electronic control box since the older trans wasn't set up for electronic controls. I think a box is about $100? A good mechanic should be able to install this trans. The drive shaft will need shortening. A good drive shaft shop does this for not much. Or, you can get a Gear Vendors overdrive unit. I think they're about $2k? www.gearvendors.com
. Labor to install, drive shaft will need shortening. Either way you go when you have the drive shaft off it's a good time to replace the u-joints, get both shafts balanced and checked for straight. Gear Vendors units are very strong. They could probably tell you what the rpm reduction would be for your particular application. I put a Gear Vendors in my Vette. It drops my rpm's about 500 when cruising at 70 mph. I don't know if you'd be able to figure out accurately how much less fuel you would use with a 500 rpm reduction? Best would probably be to find people that have good mpg data before and after putting in a Gear Vendors.