Lowering the rear on a '57 GMC

[Riggs'57]

I just saw a bracket on ebay that allows a person to flip the rear axle on a truck without relocating the axle tube perch. I'll post the link below. Does anyone think this will work on my '57 3/4 ton truck? if not, I'll weld on new perches. I've never done this before, I don't know what angle to set the axle at. How do you do this? Also, is c-notching compulsory for an axle flip?

http://cgi.ebay.com/ebaymotors/73-87...spagenameZWDVW

Thanks people!