Have you checked for a ground wire from the body/sub-frame to the engine? Some of the Z's had a ground strap between the tail of the transmission and the rear transmission support. It may not have been replaced when the last owner replaced the clutch. The ground wire on the battery that goes to the engine block should also have a small 10 gauge wire (pigtail) that should be attached to the firewall.
Sears makes a handy little tool called a screwout that has a left hand bitting/cutting edge that bites into whats left of the screw head and turns the screw out.
It looks like the hose clamps have a 5/16" hex head. Get out your WD-40 spray the fitting. Give it a few minutes. Then get a 1/4 drive socket ratchet with a 5/16" socket and give that a try. Some of the older hose clamps had a 3/8" hex head screw.