The exhaust port in those heads is raised 0.100".  Raised exhaust ports often cause header fitment issues.  It's less than 1/8", but it can be enough to cause the headers to hit things they ordinarily wouldn't. 

I can also tell you that the headers are designed for 4V heads and the heads you have are based on 2V ports.   

Furthermore those headers don't claim to fit a Mustang; they're for mid size Ford/Mercury vehicles (Fairlane, Cyclone, Ranchero, etc.).  The correct part number for the Mustang is 6210-1HKR.  That said, there's still no guarantee of compatibility with the heads, because these are also listed for 4V heads and I can't say what effect the raised ports may have. 

It sounds to me like you have mismatched parts.  I know there are a couple guys here who have Clevelands, so hopefully they chime in with specifics as far as what will fit.  Welcome to the forum. 

Thank you for your reply.  I had the Mustang shop order these headers so it may look like the wrong ones were ordered.  I have called Trickflow and they have assured me that both 4v and 2V headers will fit
Is there a way that my mechanic can contact you directly?