Aftermarket carb for 170?

The two barrel intake is the only way to upgrade the slant 6 to a two barrel that will really reap any benefits. As mentioned, the bottleneck of the one barrel intake is huge. Also, unless you upgrade the exhaust at the same time, there's really no sense in doing the two barrel. That means either headers, or having the stock manifold ported and having the outlet opened up. I have a stock manifold on Vixen, my 64 Valiant that's been ported and the outlet opened up to 2.5" and it added quite a bit including other mods I did. The slant 6 needs a complete, systematic approach, not just one thing at the time like a V8 to see real benefits. Otherwise you'll be spending money seeing very little if no improvement.