I don't see a bypass on the crossovers - just an inlet - so I don't think you can run a T-stat with those.
Unless of course that little nub on the front unthreads and you can get a fitting in there and run a bypass straight to the top of the t-stat housing.
If You do end up running a t-stat it's advised that you drill some holes in the T-stat to keep water moving through the block at all times. If you go that route, it's advised then that you also have a pressure relief valve installed to avoid over-pressure situations when the t-stat is closed.
Here's how mine's plumbed.