How accurate is your 7400 right now?
I own one in 30-06 that with the right ammo will shoot under 1 MOA (Rem CoreLokt, Federal Fusion). However, other ammo can be 4-5 MOA (Remington AccuTip). This is all using factory loads.
I was just thinking to myself... if you really want to use the 7400 that's fine, but note that a $500 scope will be worth more than the rifle.
If it won't shoot under 2 MOA (which you really should have for a 350 yard shot) in its current form then I'd seriously think about a different rifle. You should be able to tell this using your current scope.
I'd hate to have you buy an expensive scope and find out the rifle won't shoot well. Some 7400's have that reputation.
Keep us posted.