I recently purchased a rust free 68 D 300 which has had only one owner since early seventies. It has 52,000 original. . Strong 318 with NP 435 4 speed. Love this flatbed and the way it drives.!! ( up to 52 mph )

I want to refresh this D 300 and it will soon replace my existing newer dually.
This D300 will be my #1 tow / & over all hobby work rig.

I want to achieve more final drive ratio than the existing ratio of 1 to 1.
It presently feels like 3200 rpm for only 56 miles per hour.

I like the 2nd,3rd,& 4th ratios that I now have, so changing out the ring and pinion will not give me what I am looking for.

If I end up with the five speed it may calculate out to a need to a change of rear ratio. ( maybe,.. Maybe not )

I would appreciate opinions and ideas of how to increase final drive ratio.

I offer a couple 3 ideas to throw out there.

1... Do I hunt down a five speed out of "what year"? and "what model"? to bolt up to my bell housing? Pilot shaft depth etc.?

2... I am reluctant to consider a 2 speed brownie due to finished product and overall cost as well as my man hours ?

3.A. Two speed rear axles,.... I refer to the ones with the electric small motor inside rear axle housing,.... Certainly not one of those 2 tons or larger, with huge wheels,..

3.B.. However a foreign smaller dually? with less rim size and lugs than the large 2 + ton? I do have history on these electric 2 speed axles.
I definitely would consider a 2 speed electric axle setup if a match could look and perform well. Price constitutes a used assembly. I don,t know of any models "foreign or domestic" that are “one ton” compatible.
Maybe you fellas might know of a potential 2 speed axle that may be a potential transplant foreign or domestic.?
On these or better ideas, I have need of some.

*** I do have a Strong built 360 in storage yet this 318 is surprising.!!
*** The tire size is L78/16 LT and the axle ratio is 587 Spicer 70
Gordo