John, do yourself a favor and DO NOT use the port on the radiator for your temp sender. Since you are running a small block Chevy, place it in the intake manifold's water crossover. This will give you the coolant temp BEFORE the t-stat, which is preferable to after. Think about the scenario of a stuck closed t-stat. Temp sender in block, heads or intake will show true engine temp while one in the radiator will read lower and possibly not show a problem exists until much later. Been there, done that with my 100 many years ago.
You can get a cap for the port from some of the usual sources. Just have to look it up under the six cylinder radiators. The threads are British Standard Parallel Pipe, if I remember right, but I don't recall the size off hand.
Here is a link to the cap at Ahead4Healeys-->
https://www.ahead4healeys.co.uk/BLANKING-PLUG---RADIATOR-id1407.aspx