Best trans combo for a CRX DX?

[GasSavers_Hal9000]

Any Honda gurus, I'm really hoping you'll chime in on this one, but everybody is welcome to comment of course.

OK, I've just picked up a replacement (manual DX) trans for my '88 CRX DX (auto), and I have an HF final drive on the way that I can swap in to really leg the trans out if needed.... BUT.

To date, all the indicators show that I'm getting the best FE at around 3300-3500 rpm. With my auto, that puts me at around 73-76 mph or so. I'm assuming that the stock DX manual will have a similar final drive ratio. So will it really serve me well to add the HF final drive and drop my freeway rpm down below that point? I haven't crunched the numbers yet, but I'm thinking that the only way it would pay off will be if the HF final drive will put me in the butter zone in 4th gear and leave 5th for the flats and downhills where I can let the rpm drop when the engine isn't under any load (I live in a very mountainous area, which complicates my efforts some)... Is that in line with what other people are doing?

Also, does anybody know the most efficient rpm of the D15 VX engines? I'm giving some serious thought to picking one up eventually to stuff in my CRX. Maybe the HF final drive would be a better mate to that than to my stock engine....

Thanks in advance for your advice everybody....