You are correct. Have to take the freeze plug out and put a fitting in there. The fitting has an internal and external o-ring on it and there is a small rectangular plate with one corner cut off it that slides into a groove on the fitting to hold it in along with a short 1/4" bolt. This fitting was very common on G.M. trans for many years. The powerglide, TH350, 3 spd and 4 speed, both Muncie and Saginaw used the same fitting and gear. The TH400 and M21 Muncie used a much larger fitting. Just try it with any gear that you have and you can fine tune it later. A good rule of thumb for these gears is that every tooth, more or less on the gear will make about 5 MPH difference on your speedo. Hope this helps.
And by the way, excellent job on the Coupe. I did the same to my 1940 Chev 2 door (that I still own) back in the early 80's but took me 5 years to do what you have done in 5 weeks. Boy does that ever make me feel older and slower...lol

Ontario Inliners
1965 Chevelle
1940 Chev
1965 Chev Pick-up
1970 MGB Roadster