Bought a set of x heads for my 340 for my 71 340 dart restoration project. Wanted either big valve j heads or x heads. So had these heads for a year and finally sent off to head shop. They pressure tested and found leaks at the valve seats (all seats had deep seats installed at one point). So found a set on ebay for 275.00 for bare head cores. The fellow said he had them hot tanked and checked at a machine shop. I get them, someone has ported the exhaust ports and one head fails the pressure test just like my others.My question, does anyone repair the valve seat areas?? thanks.

You should be able to put hardened valve seats in them. People do that all the time to run unleaded gas. Very common procedure. If your machine shop doesn't know that then you should find another machine shop. I'd have the guides check out too because they might need to be replaced.

Talked to two machine shops, one is called heads unlimited. All four heads I have all have had the seats replaced and this is where the problem lies. When the heads are pressure tested they are leaking from the seat area. thanks

and the seats can be replaced.

what EXACTLY is the problem

I guess who ever did the seat install used a deep seat which went into the water jacket. I found a shop that can braze the problem area but says it is very expensive.