The “Real Z/28” means nothing and carrys no extra value for the car because the original engine and trans are gone. It’s now just a 69 Camaro that’s been slightly modified, so you price accordingly to similar ones on the market. Provided it passes the sniff, drive, and acceptable body inspection, $50k should easily leave room to negotiate. Regardless, if you can get the car on a lift, pay the shop for a thorough and underside inspection….. do your due diligence.