For long tube headers, what fits in a '69 will not fit in a '67. I can never remember if a '68 is early or late style.

The automatic early trucks had the same rear crossmember but it was cut and the middle section replaced with something that looked homemade. It is possible that enough material was removed that the collector does not run into the crossmember.

I just know on my LA powered '64 there was no way to use headers without major modifications, like lengthening all four tubes.

Moparts does have a truck forum with plenty knowledgeable people.

R.