I bought long tube headers from Clifford Performance for my 1969 C20, 250 project. The headers did not fit. The tubes of the front part of the header hit the bottom control arm. I called and emailed Larry and he insisted that they should fit. I sent pictures showing that they hitting the control arm preventing them from lining up to the engine. I even trimmed the motor mount for extra clearance. He offered an exchange for shorty headers, but I was again concerned about fitment. He finally agreed to take them back. I had to pay return shipping ($106). After a week of emails and phone calls I eventually got my $399 refund minus his shipping ($40). So, for $146 I got a headache and heartburn from Clifford Performance.

Sorry to hear of the problems.
Have you thought about using the Dual Cast manifolds?

I went with a 292 manifold just cause I knew it would fit.