Angle board first.You will be taking the flat board out ten times to one of the angle board.It sounds like the welting you are using might be too thick.Years ago I used felt stripping for house windows.It was a little thick,but it broke in to fit after a few years.Years ago I made some floors out of some real high quality plywood I had bought at an auction.It was a true 3/4 thick,maybe even 13/16.With welting I really needed closer to 11/16.I used it without welting in an A pickup in 04,now I've kind of forgot about it.I also used it for the floors in a Kenworth truck,but they're not too fussy.