Willys Jeep guy just brought home 66 fleetside

[66fleetside]

Spring arrived and the though of working on a 66 long bed from Texas versus my 52 Willys no floor fred flintstone truck won out.
So far cleaned out the mice and removed the bed as wood was gone but metal is intact. Neighbor suggested 3/4 pressure treated plywood?
My issue is with the straight 6. The motor seems stuck as a quick attempt with the fan belt showed. Strangely enough though the crank pulley has play to the tune of about an inch or so. I removed the belt completely tonight and could see the crank nut not moving and the pulley seems wobbly and broken. I am thinking of putting some brake fluid down the cylinders to see what happens. How can I tell if it is a 250 or 292? Where are the numbers and can I find out if the motor is original. I see Autozone and Advance both have crate engines for about $1500-$1700 bucks.