cruising revs in a 1500 midget

[spitfire4v8]

Mrs 4v8 has a 1500 midget .. I hadn't appreciated just how low geared these things are!
I've looked around at the 5 speed gearbox conversions (and whinced at the cross member being chopped out ..) , but with the 1500 engine first gear is too low anyway, so a more cost effective way forward would appear to be a higher diff ratio and move all the gears up a notch in ratio terms.

However, diffs of 3.7:1 seems to be as far as you can go (putting aside the ultra rare 3.5 ratios anyway).
It seems that only the last midgets got 3.7 diffs and ours is very probably a 3.9 (1977 vintage).

Has anyone changed to 3.7 from 3.9 and was it in any way worthwhile?
Does anyone make a set of aftermarket 3.5 gears nowadays?

any help / advice appreciated cheers

[spitfire4v8]

Hi yes I'd heard that the overdive box can be made to fit .. potentially a cheaper alternative than going for the ford 5 speed box too now that those seem to be creeping up in price. cheers !