Whoever removed it and welded the holes up may have thought this was a good idea, but they didn't do you or your engine any favors, Ed. Sorry to be the bearer of bad news, but it is the truth.
How to fix it?
1...find an exhaust manifold with one still in place and make it work properly again.
2...I've seen people adapt the type used on Chevy small block V8s and some AMC engines. These are different. They fit between the exhaust manifold and the exhaust pipe flange. Like this: https://tinyurl.com/da8znwn9
(but there are some used on AMC engines which had 2 holes...like the 235...and those have a better chance of fitting). A few years ago I watched a guy make a little 3 hole adapter so he could use the Chevy SBC type on his 235 and then he just used the 3 hole piece on the exhaust pipe.
3...if you had the patience and skill, you could drill your old manifold out (with a drill press) and make a new shaft and flapper and get a new bimetal spring.
Whichever method appeals to you the most, your engine would be happier with that flapper in place.