One thing to remember when swapping over the HO head for the Renix: The ports on the HO are a different height and to maximize the swap you need to also swap both manifolds, or do a port, and polish, job. And check gasket alignment, They're notorious for a sloppy fit.
On the gear ratio the way the Big trucks do it is first find the engines torgue peak. Say 3200 RPM. Then take the speed the truck will be travelling, 90 MPH,,er,,I mean 75 MPH. Now you select the GR or TS that will let the engine achieve it's power peak in it's highest gear. However you don't see too many 18 wheeler's rock climbing, but the basics are the same. Example. I have 2 '89 MJ's One is a 2.5 with 5speed and 4.11. The other is a 4.0, 5speed and 3.73. I notice no difference between the two on take off or doing the hwy at 75. I'm sure with a load it would make a diffrence. My 2 cents worth.