Hi Dave,
Thanks so much for posting this thread, and for going through all the trouble to get it sorted out. We just purchased a Blueprint 427 EFI and Johnny says that we have the same head as your 347 so it looks like we are in the same boat. We also want headers with a collector for an O2 bung so that we can sample an entire cylinder bank and we also do not want to put that in the side pipe.
Based on these requirements, the 4 into 4 FFR headers don't work (no collector) although it sounds like they would fit. It also doesn't look like the Gas-N ones have a collector (correct me if I'm wrong). This leaves the GP headers which sounds like is what you ended up going with.
Did you have a specific contact person there who understands the issues? I'm planning on giving them a call and want to talk with the right person...
Thanks again!
David