Hyup that's a pretty big 42% increase in rpm at any particular roadspeed.
Most overdrives are in the range of 72% to 78%, so that will get you a NET change back to close to what you started with, in other words your 3.92s will final drive at say 3.92 x.72 =2.82
The Mopar OD ratio is .69 so; .69 x3.92=2.70 WINNER. lol.
IIRC the chevy is .62 so; .62 x 3.92=2.43, so your engine better be able to pull down there; as 65=2000 or less. And it is sometimes extremely difficult to give the engine the ignition timing it wants down there.
IMO 2400 is about the lowest rpm you can cruise at and still make a decent timing curve, with a decent cruise rpm.
