Why not take the manifold off, and have a look see if you can move the guts of the 49 to the 41. If not, I'd just run the 49 until you find a "new" 41.
I've driven cars and trucks with a stuck heat riser. If you move the valve to "not-heat", then the old bus will run okay, but it will take forever to warm up, and on really cold days, it wont run as well. There is a reason Mr. GM put those darn things on there.
The heat riser valve on my 235 was stuck on "heat-on" and after 5 or so minutes of running, the old beast really suffered. I moved the valve to heat off, and then eventually got it free, and the difference is quite dramatic. I also enjoy hearing the valve clang shut after I shut the truck down, and it cools a bit.
