This replacement MGB heater valve is very simple to make. Of course, you'll have to operate the valve manually from underneath the hood with this modification, but by using the fresh air vent or rolling down the windows it's not that hard to get rid of excess heat in the few months when you'll want the heater on at all (conditions may vary in your location). It's easy to change back to the stock heater valve in the future, and uses a part you would have just thrown out anyway.
First, I took the old leaky valve off, and using a hacksaw, cut though it:
Next, drill out the hole with a 7/16" drill bit. Thread the hole with a 1/4" NPT tap and fit the parts shown in the photos.
Note: NPT stands for "National Pipe Thread". It is totally different than "regular" bolt threads and is specific to plumbing fixtures. The outside dimension of the pipe nipples is really 1/2", but 1/4" refers to the inside diameter of the pipe. All fittings used here are 1/4" NPT, with teflon tape wrapped around the threads to prevent leaks.
[Ed. Note- You should use a barb end where the hose slips on (like the one pictured below). If you put a hose over a thread, it may leak under pressure.]