slant six exhaust manifolds
A.T. PMed me about this and I explained my reason why I have not given much info. about porting factory manifolds, it's because I never spend much time doing that. Here are my reasons:
A factory 1 bbl intake has way more port volume and passage flow then the 1 bbl carb. The port opening is not a restriction.
For best fuel vaporization and suspension, it is nice to have the intake manifold ports / runners smaller then the head port and valve pocket, that keeps the volocity up thru the runners.
Having a "step" on the floor of the intake, where it meets a larger intake port at the head, helps get liquid fuel that is running along the manifold floor, back into suspension. The mis-match acts like a "ski jump".
With factory exhaust manifolds, spending more then 30 minutes on one is a waste of my time. Reach-in with a burr and knock-out any "rough stuff", then break the sharp edges at the port openings. Spend more of your time on resurfacing and doing a careful installation.
We all know that the factory exhaust manifolds are limited by the simple fact that all 6 exhaust 'pulses' have to make their way out one, 2 inch opening. We also know that it is highly likely that the factory exhaust manifold will warp, crack and break sometime during the life of the car so keep this in mind as you spend time on it.
If I were changing a manifold set (or gasket) I would look at installing the lighter aluminum intake manifold or at least use a 'large hot-spot' cast iron intake. For MPG, make sure thr heat valve is working.
Install the manifold set as high on the head as possible and use a large exhaust pipe (2 1/4 works well)