[19Fordy]

Flathead Jack has thermostat housings for the 8BA that will match the stock 40 radiator outlets. Pricey but pretty and eliminates the need for hose reducers. You could also make and braze some collars onto the stock 49-53 cast iron thermostat housings to increase their OD. http://flatheadjack.com/78.html
When I installed a 50 Merc into my 40 I used:
49-51 Merc rear sump oil pan. Provides more clearance for drag link if you severely "rake" your car.
40 column shift trans. I think the 39 trans. will work also.
49-51 Merc flywheel, pressure plate and clutch. Clutch is 10 in.
40 throw out bearing
8RT wide belt water pumps
Wide belt generator pulley
Stock 40 exhaust manifiolds
49-51 Ford pickup 2 sheave pressed steel wide belt crank pulley
Stock 1940 Ford engine mount biscuits
49-51 Merc pressed steel bellhousing. I think early Ford pick up truck cast bellhousing will also work, even if you use a 10 inch clutch and pressure plate.
I welded "ears" onto the Merc pressed steel bellhousing to accept modified anti-chatter rods. Make sure they clear the clutch and brake pedal when depressed. or, you can make an after-market anti-chatter device that bolts to the Merc belhousing. See photo.
To have the pickup wide belt pulley clear the front cross member I removed the stock "Y" fender support bracket and made new angle iron support brackets that bolt to stock holes in the frame. See photo. Also moved the radiator forward and made new radiator support brackets to attach to the sides of the Walker radiator. (PS: If you decide to use a new radiator, Walker will make you a radiator with smaller top inlets to match your thermostat housings.) Also, disregard notes on crank pulley photo. By far, the most troublesome part of this engine install is respositioning the fan so that it will "clear" the top radiator hoses and still provide adequate cooling. A search may reveal alternative ideas. I ended up making a new fan carrier bracket and fan pulley. You need a lathe to do this. It's a lot of work. Consider an electric fan. I did this install in 1970 and it's still in the car. Hope I didn't forget anything you may need to know.