help merc 260 sterndrive, alpha one gen one lower replace,

[caribbeanrumrunner]

the prop clanks when in gear i can spin it freely , i just put a new alpha one lower unit on ,i did it while in the forword gear both boat and unit, now when the boat is off i place in forword or reverse and i can spin the prop and it clanks loud , but in nuetural it is fine it spins free,, verry frustraded, the reason for changeing the out drive is because i lost reverse for nailing a stump at 60 mph, . now im frustrated , i can put the gear in forword and it spins and clanks but if i put the throttle all the wat down the prop crabs fine ,