jb, I think that Randy's on target here, and just to clarify when we look at the Gearvendors site and run down their list in the various transmissions that they list we're selecting the right Gearvendors unit to match up with the tranny we have in the vehicle. Are you considering changing out the tranny, and then adding a Gearvendors unit behind it?$$$$$? I'm a bit confused with the options you're considering.

I'd think that your 6 liter Vortec would be a lot happier cruising at right around 2000rpm when at your desired cruising speed in 5th gear, plus you'll be up on your torque curve more where you'll have some power for the hills & curves without downshifting. Lugging it down to 1200-1500rpm is going to be hard on the lower end, and will tend to load up on you over time.

I've got a little spreadsheet that I built for my Tremec TKO that let me see the speeds in each gear across the rpm range, based on the tranny ratios, rear end gear, and tire circumference. The real test is what feels right in the vehicle, selecting your shift points, but it helped me to get a feeling for what I would experience based on the numbers, before spending a dime. Like Randy asked, what's your tranny, and also what's your tire circumference?