I have a 78 Cadillac it has no power when I put it in gear

Is it also running hot, possibly boiling over when you shut off the engine? Is the transmission shifting late? If so it's going to be a worn timing chain. Check your ignition timing to be absolutely sure. If it's retarded (late) the timing chain and sprockets will need to be replaced. GMs used plastic camshaft sprockets in those days that tended to wear and fall apart around 150,000 or so miles. Hope that helps! Jim