You say that you have a 3 speed w/ granny gear. That is a 4 speed, not a 3 speed. SM-420 in Chevrolet speak. As far as add on overdrives for $200 bucks, good luck. I've done it, but more by luck than by common availability.
The way I did it in my '61 K10 was to purchase a Spicer 2021 over/underdrive and put in in between the the trans and the transfer case. Super short driveshaft. It works nice. I bought 2 of them. The first was $300 bucks and it howled like Banshee when at speed. I found another one on eBay and paid $200 foir that one. They are not common. Go ahead and search eBay right now. I'll wait.
I didn't think so.:)
You could use a lever to engage, but I went with a PTO pull knob and cable as it was less destructive to my original truck.
The OTHER way to get overdrive with an SM 420, is to get a Ranger overdrive. this unit goes between the trans and engine. It moves your trans, and thus your shifter, back 7 inches.
Info here for Ranger OD. Summit carries them for just under $1,400 bucks. Not cheap, but not terrible either.
Frankly, I don't think a T-5 or Borg-Warner is going to stand up to a 4x4 drivetrain. B-W's are not particularly strong. And t-5's are going to push your transfer case WAYYYY back. I not sure how long the NV 4500 is but I suspect that it's longer than the sm420.
Here's mine. Just in case you wanted to see. It will hit 90 mph pretty easily. All factory drivetrain other than the Spicer 2021. 6 cylinder.
