55modified
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Though you say you are replacing the "thermostat" with an SW I wonder if you mean the WT gauge. If so it would be very simple to leave the present sensor in the radiator in place and use an electrical gauge with a sender mounted in a short section of metal tubing spliced into the long hose that goes from the lower radiator nipple to the water pump, with a wire from the metal section to ground.
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
It's either a 5/8 or 1/2" bpt I decided. Not sure which yet.