If the measurement is the outside diameter of the threads, then I think they are 3/8.
Pipe threads and their respective sizes can be very confusing. However, if the diameter of the threaded portion of those plugs is about 3/8", then it is a 1/8" pipe plug; if the diameter is about 1/2", then it is a 1/4" pipe plug; if the diameter is 5/8", then it is a 3/8" pipe plug. This is pretty valid for both US and British pipe threads. Yes, there exists other differences and these dimensions are not exact, but that is not important here.
The 90 ell fitting which Salvatore used on the tube fitting below your boiler's safety valve likely has 1/4" BSPP pipe threads in the boiler. It is SIMILAR in style to what I suggested for the bottom plug. Unfortunately, it does not look like the required turning radius is available. Even if you extended the fitting away from the boiler by using either a short nipple or a close nipple, you would be very close to the rail.
As far as the 200 F rating on the valve, I missed that and, without doing a little homework on the valve, I suppose you would be taking a small chance even if you could fit it in. I have, though, retrofitted that very same valve as a drain fitting on an Astra espresso machine and it has been working just fine for well over a year.