I have a 79, installing RB Header. Need to bolt header to heat exchanger. I swear the pitch is different to the metric nut I am trying to use. So, what nutsize should I be using?

 

10 mm x 1.50 mm

 

The heat exchanger is probably larger than your typical exhaust stud in the engine. What you want is a size that is more likely 12mm x 1.25 or 1.00 which is rare and can be potentially difficult to find in a proper material to withstand exhaust heat. The factory nuts are the best choice if you can get another set. See about asking in the FS/WTB section.