Exhaust drain screws are M8x1.25 x 10mm
Probably too small for a petcock. it's equiv to a 5/16" dia screw. You could remove manifold, drill and tap a 1/8 NPT pretty easily. You would not want to put the petcock on the manifold as there is not clearance. You could put a pipe fitting there, then run a 3'8 in hose from each manifold, from the power steering cooler (you'd have to drill that too) from the block fittings if they still work, from the oil cooler and drill and tap your thermostat housing (put small petcock there). put a T fitting in your heater and shower connections. Run 20' of hose from all points down to the bilge to a manifold block with one big petcock. Or solenoid valve. Voila, and you could even put air on the tstat housing to push the water out of the engine, You can pour coolant in from the heater hose stubs on top of the block in the back.
You'd still have to drain the sea strainer, but that's everything. It takes me less than 30 min to drain my engine so too much trouble for me:)
