My current 1988 340 has a 4 speed box and is very un-economical, is it possiBle to put a 5 speed box in it and would it make it any better on fuel? Also would my insurance go up? Im assuming I'd have to notify DVLA aswell?

Yes it's pretty easy but you'll need to get yourself a 5 speed diff too (it's shorter physically). If you want to be on the books, notify the DVLA but tbh man I don't think anybody is going to mind too much, as long as you tell your insurance (not that they're likely to notice post-accident anyway).

Push comes to shove just tell them it was a standard replacement part for a broken gearbox.

I doubt it wil make alot of difference, the 1.4 should be pretty economical if driven gently and on the motorway. What are you getting?

Check:
Air leaks
Carb mixture + general carb condition (breathers clear + intact)
Good clean condition of plug/leads/cap/coil (spotless)
Good condition of air filter + exhaust (Volvo original?)
Valve clearances adjusted properly
Good compression

Im currently in the process of swapping my gearbox Its a fairlyyyyyy easy swap The 4 speed has a longer diff. When put side by side, the 5 speed has an extra section that goes between the diff and the gearbox, this holds 5th gear. And so the diff is then made shorter to make both the 4 and 5 speeds the same length - does that make sense? Just remember the most important bit, the new gearstick with the 5 gears on it

Hi mate.
I have just done this conversion. All the bolt holes are the same. Its a streight swop.
It does make a difference haveing a fith gear. I have kept the origionall gear lever.
Its a secret fith
Cheers Mark.