I would buy at least a 600 CCA battery. Buy the largest CCA battery you can find if you live in a cold climate.
Ideal is 14.2 volts 13.8 is acceptable
The back seat should pop out and your battery and some fuse boxes are under your back seat.
If you unhook the battery for an hour, when you re-connect the cable it should reset the computer.
Linked is the diagram for a 1976 Cadillac DeVille, 1977 should be roughly the same...
Go to rear bench seat. Pull up on front of seat, can be tough. Seat should pop out. There is the battery. Unhook cables, and remove bolt near bottom of battery, which holds a bracket to keep battery in place. pull battery out.
You should check out the Cadillac Deville suspension conversion kits at strutmasters.com If you can do brakes you can install their conversion kits yourself. http://www.strutmasters.com/cadillac-deville-suspension-s/42.htm
193 to 210
your local auto parts store should have a guide to replacing. or look in your manual
Yes, it should but watch out for different features in each door like heated rear seats etc.
A 1990 Cadillac Deville air conditioner should hold 46 oz. R12 Freon. This should be verified through the owners vehicle manual.
5x115 Center bore should be 70.3
unhook battery for a few minutes that should do it otherwise have an auto prats store turn it off with a code reader or diagonstic tool