Gear Vendors overdrive

having an OD will be pricey any path you choose.

i've never heard bad things about the GV overdrives. you can also switch to a more modern trans with OD/Lockup converter (whether mopar or gm) but both will also require cash on hand; they might get slightly better mileage because of the lockup converter, but take a lot more work getting them to fit and getting the electronics correct.

my only suggestion is that if this is an extreme build (very high torque / hp) that even 727's will start hacking. at that point you would go with a jerico manual trans setup (they are not overdrives, final 1:1 but 1st gear is very low, requiring less rear axle gearing) and they can handle all the power you can throw at them. but again you are talking big dollars. and they are not necessary if you have a mild/moderate street build. but if you have a blower, turbo, or nitrous and 12" slicks that hook hard and you are beyond the point of blowing 727's, you will want to look into them. http://www.jericoperformance.com/

if you have an extreme application that needs overdrive you would have to look at Lenco

it depends on your build.