exhaust valve size increase on J head

Stock manifolds will really limit what you can do with a stock stroke 340. On a stroker they will definitely kill some serious power. I wouldn't even consider it. I have stock manifolds on my stock stroke 340 that makes around 400 horse and I am SURE I am losing power. I won't be surprised if I pick up 20 horse going to headers. 1-5/8 full legnth headers will give you the best power up to the 500+ horse range. I'm curious why you would want to work on only the exhaust side when you have 1.88 intake? 2.02 intake is a known power maker. 2.02 and 1.60 work great. Clean them up. Cut them proper. Do some gasket match. And you'll be more than pleased with the results. The one area that really needs attention on these heads is the turn in on the intake side. Smooth that out and you get much better flow and less turbulence.