will a 5 speed gearbox from a T3 fit directly into a T2.5 in place of the 4 speed ?

Whats a T2.5?
if you mean the air cooled T3 from '80 to 83, no it won't, the 5 speed is longer than the 4 speed.

Why would you want to anyhow, the 4 speed is a much stronger box.

Thanks Dave
I was told that T2.5 was the correct term for the aircooled '80-'83
Was hoping to lower engine revs on trips

I have an 83 T25 with a 5 Speed in it. It is water cooled 1.9 though. Is yours air cooled ?

yes, mine is air cooled