I have a stock Peugeot 103 sp 1979 here in the uk, only mods are a polini reed and baffles removed from the stock exhaust so I had to upjet a little so temps may be slightly higher than fully stock but not much.
I purchased a trailtech head temp guage which attaches like the prev poster mentioned on the spark plug.
you wont get an accurate reading with anything else when moving.
its very accurate and I record temps of about 300f cruising and going up a big hill it rises to a max of about 340. idles about 250f
I would expect seizure territory about the 400-450f mark but never got anywhere near that, highest I have recorded is 362f up a very long hill and I am 16 stone.
hope this helps