mercruiser 260hp won't crank

i got a 350+.060 with 305 heads,1500 miles, last summer,i cranked it with the spark plugs out and put oil in the spark plug holes to keep it for the winter,this spring i took off the 305 heads to put 96 vortec heads on,found that the head gaskets were blown,some rust in #4 and #5,cleaned the rust with a scrubber pad and removed the water,put dry gas in those cylinders and blew them out with hp air,then oiled the walls,cleaned the head gasket surfaces and scrubbed with pad,cleaned with laquer thinner,put the marine gaskets and vortec heads on by the book,intake,carb,etc. the valve are not tightened yet,now it won't turn with the starter,put a pipe wrench on the steering pump with pipe extention,crank only turns back and forth 10-15 degrees.been spraying pb blaster penetrating oil in the cylinders...any suggestions???
thanks very much

Pull motor, tear down and redo it..........dont waist time with snake oils........

Was not winterized or due to blown head gasket water got in........

You say the motor is froze, is the drive froze also ? Remove the drive turn it. not turning then you had to yank the drive anyway to remove the motor.