Auxillary overdrive O/D

I've looked into regearing but I was only able to find 3.55 gears which with the cost of the parts and labor, it isn't worth it from the 3.73s. I don't want to put a different transmission in it so I'm wondering how the gearvendors overdrive is or any other recommendations. Right now I'm putting out about 375-400 horse and somewhere around 800 or so ft lbs. I want to add a bigger turbo on top of the he351 but I have to address the problem of getting the power to the road and shifting at 12mph, 25mph, 38mph, 53ishmph, then into 6th which on the highway at 75-80 I'm around 2500-2600 Rpm