I'm not sure it's worth the effort to put a temp gauge on, as the thermosyphon system is quite efficient, as long as the coolant is at the proper level. You'd have to work it very hard on a hot day to even worry about it. But if it makes you feel more comfortable, knock yourself out.
I pulled a 4' rotary cutter mowing 4' tall grass years ago with my Farmall A on a 90º day. It vapor locked before what I would call "overheated". Mainly from heat generated from the exhaust manifold. I remembered an old trick Ford guys used on their cars and trucks from the early to mid 60's. Went to the house and got 3 spring type clothes pins, put them on the fuel line and it ran fine the rest of the day.