Speedy88 wrote:340 uses a longer diff rather than longer prop, why didn't they just do the same with the 360?

The 360 is meant to 'better' with the torque tube and no nasty coiling up of the components as the high power 340 guys have, but it does add alot of extra weight.
Mark, the 360 diff will be a lower ratio, meaning harder for the 1.4 to drive the car, but not sure how much difference it would make. Personally I wouldn't bother unless you break the 345 diff, I broke mine, they are weaker before 1982, but then I was trying hard. I don't think it will save as much fuel or quieten the car as much as a valve grind will. Those valve seats are pretty soft I think.
Edit to say - Strangely the B200E hatchback diff is 3.64:1, which is the same as the 84-on B14 diff. Your 345 diff is 3.91:1 apparently. So I'm saying its probably fine, but I still wouldn't bother. (presume you're eyeing up you old gearbox+diff from the GLT)