Oops!
I wrote my previous post last night, had a beer or two, went to bed, and whilst awaiting sleep thought "Bugger! I didn't put enough information in that post!"
Tyre size is 225/75 R16.
Engine is Chev 400 ci Vortec, purchased as a new crate motor 2013.
Gearbox is 5 sp manual with electric speedometer. I've crawled under the bus, gone through my invoices, but can find nothing to actually identify it so that I could identify the ratios in it.

At those revs in top (5th) gear the engine isn't labouring and is very responsive, put the boot in and it launches instantly, there's absolutely no lag...it feels right.
For sure, I have to flick it between 5th and 4th fairly often; but that's not a problem, and at those revs in 5th I've got fuel economy.
So I'm happy with revs/ratios in top gear, it's 2nd/3rd, and 3rd/4th that is the worry.
That's the reasoning behind wishing to split those shifts, and still retain the lower revs in 5th.