The MBT-70 tank project was pretty advanced for it's time, a bit too advanced it seems as it 'reached' too far and never got to production.
POD - Lets say the Americans followed the lead of the Germans early on, sent the 152mm gun/launcher with that pesky caseless ammo back for more R&D and put a conventional 120mm gun in the MBT-70. It just squeaks by Congress and goes into production.
As production/overhauls on worn out tanks go on improvements are made to systems to bring them to a similar state as the M1A2 SIP. I speak of the digital combat systems, optional TUSK, etc.
How would this tank make out on the mean streets of Iraq?
BONUS QUESTION!
How would sending the 152mm gun/launcher program back to R&D effect the M551? What other weapon could have been used on that vehicle?