Rear axle swap (7 1/4 to 8 3/4) - driveshaft?

Howdy! Just yanked the stock 7 1/4” from my 65 Barracuda and am transplanting an A body 8 3/4”. Behind the (mostly) stock Slant 6/904.

Q1: how much does the driveshaft need to be shortened? I saw a formula a while back I know it’s just math, but can’t find it now.

Q2: anything I need to worry about in dimensions/driveshaft angle, or will it be about the same? (I have adjustable spring perches so I’ll dial it in before welding them on, see pic!)

Thanks in advance!