Gearbox Questions

[Smiffo]

I'm sure I have asked something similar in years past, but can't recall or find the answers.

I have ( what I think under layers of thick black protection coating ) is a 5sp gearbox, code ABM or ABN.
From memory the stamp was hard to read before the coating of protection was applied, and it started AB with an illegible letter thereafter that was either N or M.

I also see about 3.8k-3.9k revs at 70mph, so that roughly fits given I have slightly smaller wheel circumference than standard.

Every now and again, 3rd gear feels like it doesn't sit home 'right' and when cold, unless I double clutch second will crunch.
All of this niggles my mind and the thoughts of a new box resurface from time to time.

The 5sp has a poorly geared first gear, which is almost pointless aside from the fact second is too tall to pull away with on a standstill ( even on flat ground. )

Seeing as I quite like the final drive ratio, and seeing as a 4sp doesn't necessarily compromise on top gear motorway driving, I have this question....

Would it be possible to gear 2nd - 5th to the same ratio as the 4sp ABB gearbox, and thus leave the nearly pointless existing 1st gear only for pulling away on excessively steep gradients or heavy towing only? 

i.e. could I have it geared in such a way that 2nd-5th are the same as an ABB 1st - 4th or are the specific gears for a 4sp not able to be installed into a 5sp case?
 

[multisi]

That dodgy 3rd gear could be the syncro hub cracking up, i wouldnt be driving it anymore, it could break apart and leave you stranded.

[Smiffo]

I've been telling myself that for ages, but I never do anything about it.
Kind'a settled on doing something now though before I start a non-urgent DJ swap lol.

Thing is, to get it serviced / repaired means being off the road unless I use a replacement for a while.

That led me to question whether I buy anything to run with for a period while I get this one resolved, or, just buy another and have the new one refurbed. That leads into which variant to hunt down, and even if just as viable to get anything and alter the gearing to my taste.