ok ... you can not use the old thermostat housing you have with the new manifolds you got, here's why, the old OMC tube style manifolds, that you are throwing away, flow different than the new one's you bought from Dennis, and no, you don't need any gasket sealer to install them. The old set you removed got the water supply from the "T" into the side of the riser, the water flowed through the manifold log to the end cap on the front of the manifold, where there are two ports, one of those ports direct water into the thermostat housing where the water is devided, a small rectangle passage goes past the thermostat, back down into the other port on front of the manifold and right back through the manifold log, then on out the exhaust. Where the water supply is devided at the thermostat housing, some through the little rectangle by-pass, the rest holds supply to the motor for cooling, and after the thermostat opens, that warm water is discharged into the manifold logs through the other port on the end cap .. so, the thermostat housing you have there only has one discharge port for each manifold, the new mainfolds you have require two discharge ports for each manifold.
You need to toss the old thermostat housing in the same pile with the old manifolds, get the one required for the new manifolds, and make you an adaptor to join the 1/2" supply line from the pump bowl to the 1 1/4" supply port on the new thermostat housing, and I would suggest putting a "gate valve" as a restrictor, in the 1/2" line also, you may need to restrict the pressure from the pump with it, as I said before, there may be too much pressure for the thermostat to open properly at wot. all this may or may not be right ya know, my brain is old, forgetful, and beginnin to hurt.
