Hi!
Today I have an issue with my left driveshaft on mine `86 360 GLT, I have a spare set from a 340 1.7, can I simply swap between them or I need to repair my old one?

I believe 1.4, 1.7 and 2.0 carb driveshafts are the same - only difference is the injected 2.0's, your GLT will have different driveshafts and I think different CV joints too

360 driveshafts are fundimentally the same, i've put a 360GLT shaft in a 360GLS, the only difference is 360GLT shafts are stronger. Putting a 360 shaft in a 340 however i couldn't say.