Hey Mike. I have the Doug's 371Y on my '70 C10, also an original 307 truck. I'll post some pics of frame clearance on both sides and the ground clearance below the frame. Years back (before I started down the rabbit hole of a frame off restoration
I was running full length Hedman's. They fit perfect, with no clearance issues, but the collectors pointed at different angles causing ground clearance problems, especially on the passenger side. I eventually swapped them out for Sanderson block huggers. I should add this was originally a 3OTT which complicated (extra crossmember/z-bar) the header selection a bit more. Also running a gear reduction starter, like the one in your post.