Swap rear ends or transmission?

Hmmm in a perfect world I'd say 46 rear suspension and 46 rear axles. No less than super40's. 411 ratio if you run 24.5 rubber. Leave the 15 in it for now. .not sure if thats in your budget.

 

What is rpm now?

New rpm for 3.9 will be rpm you have now x 3.9 / 3.55
Or for 4.1: rpm now x 4.1 / 3.55

If you're at 1450 now, as an example, you will go to ~ 1600 with 3.9, ~ 1675 with 4.1.

 

1350 seems a bit low unless you run really tall tires.

3.55 : 1350 rpm
3.9 : 1485 rpm
4.1 : 1560 rpm

 

Does that 15 have funky od like a .78 or .86 OD instead of .73

Depending on your budget you could do 4.11 or 4.33 and drop in a 18 w/ .73 od to get the best of both worlds. 3.55 and 40k rears and axle are your weak links

 

Being that heavy and 650hp 4.11 is the lowest id go imo especially with hired help driving it

Edit: if you have a little wiggle room with cash you could do something like my dad did. He build a RTXOO (technically a triple od) with .86-.73-62 OD. On his mack he runs small 295/75/22.5 tires with 3.91 and in the big hole @65 he's at 1375rpm. That way with 4.11 or 4.33 your cruising speed would still be somewhat lower. Its not the cheapest way to go about doing things tho