Slant six exhaust size?

So I won a set of long tube dual exhaust headers on eBay, and they couldn't be coming soon enough, since my tail pipe just fell off.

Anyway, I need to know the best size for my exhaust. My car is a '72 scamp, with the 225 /6. Modifications to the engine are few, but will be as follows:

Electronic ignition upgrade
390CFM Holley 4bbl (and appropriate intake)

Other things that shouldn't affect the decision will be things like a clutch fan, and perhaps some other things to pretty the engine bay up a bit.

So it's nothing major, but I've been told 2 1/4" is too much. Should I be doing 2" all the way through? Less? Thanks! (also, how will all this affect the timing?)