I wish I could help as I am wandering around in the dark kicking rocks and hating my life over this issue. I have a 72' Jimmy 4x4 with a 454. I'm on my third set of long tubes (currently Doug's Headers D303Y Tri-Y Header) that they claim will fit. They hit on the drivers side engine mount and the rear, lower bolt of the front leaf spring on the passenger side. I ordered the DIY4x.com first gen crossmember to help the drivers side issue and just because I wanted it.
I'm still trying to get the new crossmember gitup installed, but of course I have problems there too!
Is there a section on here for BBC folks to whine about our problems of things not fitting, so the smart guys can help us?