It's much rougher than you think and the decked engine is a problem. No block casting was unique to the Z/28 and a blank pad will be a deal breaker for many people. It probably had a trunk mat so check out the trunk floor, rear frame rails, roof areas around the glass. If it passes muster and the trans/shifter, axle, carb, alternator, distributor, wheels are OE it is worth probably a bit more than your current offer. If the parts in the deal are current available repro, nice but nothing special. If NOS, could be a gem or two in the pile so find out. Any paperwork with it?