I have acquired some Pace RC31 rigid forks, they seem to be 400mm, what frame would they be suitable for I'm guessing 80mm suspension corrected?

Didn't realise they made 400. Theres a 420mm which is sold as the 80mm and a 440mm which is (was) sold as the regular modern suspension corrected fork. By simple maths I'd guess the 400 is a corrected to = a 63mm fork i.e. a bit short for even old bikes today.

Good point, they are old so 63mm isn't out of the question, I'm measuring them from the centre of the dropout to the top of the crown and it definitely reads 400mm.