I would go with mid 70's to very early 80's. Once the US forces start getting TI in large numbers you'll be hard pressed to advance on them at least while on the offensive.
78-80 should be easier than 82, no TI, Abrams, Bradley etc. And only german Leo2 (was it 1979, or maybe even later) will have main gun powerful enough to kill your T-72s with ease. Watch out for TOW missiles though.
Chieftains have a 120mm too that can do a lot of damage. And the UK 105mm gun (also on Leo 1's) has a HEAT round that is just powerful enough to go through the front of T72's (depending on angle of impact of course). They'll fail to penetrate a good deal of the time too but you won't be invulnerable either so that may be a good option.