I wouldn't worry about polishing the inside of the manifold, in fact the roughness will likely help the air pick up fuel that has dropped out on the manifold walls by making the boundary layer turbulent (better mixing with the bulk flow). The roughness will also aid heat transfer to the fuel that has dropped out.

This may be a little off topic, but the golf ball example is specific to it's use and speed of operation. The dimples actually increase drag at lower speeds/ higher speeds, it just so happens that the drag with dimples is lower at the average ball speed than without dimples due to a delay in flow separation around the ball and a reduction in form drag. (https://www.princeton.edu/~asmits/Bicycle_web/sportsballs.html) You can see in the article that at very high speed (high Reynolds number) the smooth ball has less drag.

What is incomplete on the manifold? Looks like if you need a brake booster line, it is not drilled and tapped


Regarding polishing, current thinking is that a little grittiness improves flow

* Within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do

* Exhaust ports may be smooth-finished because of the dry gas flow and in the interest of minimizing exhaust by-product build-up. A 300- to 400-grit finish followed by a light buff is generally accepted to be representative of a near optimal finish for exhaust gas ports.

Suppose I didn't really need to make a thread but it may help anyone else who chooses this path.Tthe other week I saved up my pennies and bought a Rowland manifold for the A14. Good on him for keeping the A series going. Now I already knew it would be incomplete which is fine but apart from port matching, where do I start to get it to good working order? the sand casting feels kinda gritty so I'm not sure if sand blasting it would make it worse or not.