exhaust valve size increase on J head

Your best bet would be just run a dual pattern camshaft to improve the exhaust flow. The 1.60 size valve isn't the problem, you'd get a lot more out of it by just a quick clean up around the bowls & back-cutting the valves. IMO just working the exhaust without doing the intake would be foolish. Plus any improvements you gain with whatever you do are going to be compromised by using stock manifolds.