The 740 was the earlier of the three, with the 742 an "improved" version with updated stock design & some internal changes - the most important (effecting accuracy) a change to the forearm attachment. The 7400 and it's companion Model Four (a deluxe) were newer, and a little different internally, and had more up-to-date stock design(s).
It will pay you dividends to get a 7400, instead of an earlier rifle - since Remington cured many of the operational and wandering zero ills of the older designs in the newer gun.
You might want to reconsider your search for one in .308, as you'd be getting the exact same gun (weight, size, barrel length, magazine) as one in .30-06, and yet not have the same heavy bullet ability as the '06.
The very best choice in a 7400, although rare (common in the newer M750), is one in .35 Whelan.
The best reason for selecting a .308 over a .30-06, a cartridge that's shorter/lighter, is to also get a gun that has a shorter action and weighs less - exactly the reasons the US Military did so in the 1950's.
The 7400's came in two different finishes, the shiny RKW gloss, and a more conservative satin finish.
