Header/side pipes questions

[olddog]

My side pipes are designed to slip into the headers, which leak and rattle. I am going to weld flanges to both, as many of you recomended. I have looked at flanges sold by a couple companies. Both offer them with 3/8" plate material. Two machine shops and two engineers have looked at this and all said 1/4" should be adaquate.

Question #1: is 1/4" plate heavy enough?

One companies design uses 4 bolts located midway between the pipes. The other uses 8 bolts, same location as the 4 bolt plus all four corners of the plate.

Question #2: go with 4 bolts or 8 bolts?

I have EFI and the O2 is located in a single pipe in the header. I have read many EFI experts saying it is better to go with a single tube and stay within 15" of the head, including my EFI systems instructions. So, if I am welding flanges in anyway, it would be possible to make a small (say 2" long) chamber that was open for all four pipes to combine. Then the O2 could be located in this chamber where it would read all four cylinders. This would allow each cylinder to flow through all four of the side pipe tubes. I know from a tuned header point of view this small expansion chamber would be a bad idea. I think 29" give or take is considered the best length. The headers are not equal length anyway.

Question #3: would this small expansion chamber hurt performance much?

PS
Thought I would clarify: It is best to have O2 after all four pipes combine into one but only if the O2 is with in 15" of the head. Locating 30" at a header collector is often said not to work well. So they say it is better to locate O2 in a single pipe close to the head than too far away at the collector.