when you run an engine at home on the hose the water pressure from the hose is enough to keep it from overheating even with a bad impeller in the drive, chris is right check the water pump in the drive.
try alian's sales and service in cocagne new bruinswick, they have the largest supply of used parts on the east coast. www.allainsales.com or phone them at 506-576-6621, you will need your year make and serial number when you call and i never had any problems with shipping.
when this happens i found the best thing to do is to leave the key out and take some valve grinding compound and lap the flywheel to the crank before you install a new key, if not you can get arcing between the two causing a bad ground.
take the hose off that goes to the pee spigot and see if water comes out, if it does the little thing is pluged try poking it with a piece of wire. if you still have no flow pull the hose off the head and poke around in there with a piece of wire it gets pluged with sand and rust sometimes.
as far as i know you can still get the boot for your omc stringer from your omc dealer, they will probally be able to give you the spec on the carbs you need as well and you can order rebuilt ones from you auto parts store.
you would still have to change the head gaskets over to marine ones in the new engine and i would make sure that the fuel pump port on the side of the new block is not cast over as well. you should be able to buy aftermarket manifolds and risers for the volvo set-up.
i will upgrade all the valve gear as well but the edelbrok intake is made from aluminum and salt water will eat them out in less than a year, ok for fresh water only but i boat in both so i will skick with cast iron.