Help identifying size of motor

in i think in 75 they went from the 18 mm plug to a 14 mm plug so id bet u have atleat later 302 heads but could be a 289 or 302 crank in the block

If its been sitting for that long the fuel tank is probably all rusty inside. Was it stored with fuel or empty. I stored mine for several years empty and the tank that was nearly new at the time had to be replaced when we got it ready to hit the streets again. If you want to inspect the inside of the tank you can remove the filler neck and look inside and see what it looks like. If its all corroded then I would get a new tank before attempting to start or all that crap will clog up the fuel lines andn filter and possibly even get into the carb.

Some other things before attempting to start it. I would of course put new oil in it as was already mentioned with a new FL1A oil filter. I would also put new anitfreeze in it. I would also change the transmission and rear end fluids.