Taller gearing, axle or gearbox?

[Ratty]

Hi all, quick question for you;

I've recently fitted an 8 lug '87 Burb rear axle with 4.11:1 diff gears (Think you chaps call it a centre-section) into my '63 Panel in the hope of gettting better gas mileage and cruising speed but she's still a bit revvy at 70mph. Are the crown wheel and pinions in these axles interchangable with other Chevy axles (For instance Camaro) or are they specific to the 8 lug? Sure would be helpful if they are as we don't exactly have yards full of old Chevy's in England!

By the way before anyone says that 4:11's won't give me taller gearing anyway can I point out that my stock axle had something near 6..2:1 gears in it!!! Originally the truck had a straight 6 backed with a 3 speed column shift but is now running a 350 with a 4 speed floor shift.

The other option is to find a 5 speed box with a nicely overdriven top gear on it, any recommendations?

Thanks in advance, Ratty