Typically, the engine defines the acceleration while the transmission defines the top speed.

I don't expect great acceleration from this, but the top speed without a trailer I've been getting is about 65mph redlined.

If I can make 75 I'll be happy, that gives me more breathing room so I don't have to run the engine redlined the entire trip in a 55 or 65 zone. I've tried just about every engine/transmission combination that comes with the truck, but still turning up short.
I've been unsure about mods, because I don't know what was actually used on the R series,

I'm using that mod that finally turns this into an R700 with a sleeper, but its still hideously slow at top speed.