Exhaust system for a 1965 Dodge Dart GT

I guess I'm not quite getting what you're after- you asked about a header-back system, and then start talking about splitting a stock Y-pipe (which uses stock manifolds).
Do you have headers or manifolds? If headers, what brand? If manifolds, TTi offers head pipes that fit the stock manifolds. Exhaust_Parts
You do realize that whichever way you go, you need to notch the transmission crossmember in order for duals to fit. TTi can help there, too: https://www.ttiexhaust.com/Sheets/Crossmember_sheet506.pdf