Modifying headers, what size collector should I use???

I would agree with what has already been stated. 1 5/8" primaries are the size for some small block applications. 1 7/8", 2", 2 1/8" primaries are what you will usually find used on the big block applications. Time to search for another set of the long tube 4 into 1 style headers. Those tri-y's will do nothing for you.