Hi bignastygs
I assume the buick is a full frame car, like a chevelle...
you do not need rocker bars since you have a frame there, good shape I hope... and the roll cage is required to be welded to the frame and not 6x6 plates as in camaros.
Main xmember is a good idea, to tie the frame together and give you a mount for future modifications.
A dash bar is not required for a car with stock firewall and floor.
good luck