the curise came out in 76. and it does have a switch similiar to the brake switch. it disables the cruise when you hit the brake.
if memory serves me correctly they where only in the higher package trucks. but i've owned a 76 and a 77.
my best advice would be to look on lmc and get you a new filler set up.
i have a 77 chevy myself. it fixed a lot of headaches fighting with the old filler hoses.
good luck