So much of the belief system with porting is based on word of mouth and not fact. Adding larger valves can absolutely hurt power and flow on alot of street engines. Not only is shrouding increased, but the short side radius of the port will increase in number of degrees that the air has to sweep around before air enters the cylinder, unless, the new short side radius is cut way back to increase the radius of curvature and bring the ratios back to where they should be. But unfortunately, there isnt really enough meat on these heads to do it right. Increases in low lift flow should be done with proper back cutting of the valve face (I liked to grind away most of the 45, with a 30, then recut the 45 and add a 15) and enlarging the area in the throat beyond the 45 degree seat cut. Undercut stems also help. Like with most ports, the airflow is really weak on the port floor, but fast on the roof. This is because the air would rather flow in a straight line, through a hole in the roof of the combustion chamber if it could, but it cant. So ideally, you want to add material to the runner floor and raise the roof. But its not that easy, so its best to just remove flash from the floor and focus on the roof. Then cut away most of the meat that carries the angle below the 45 (the 60), opening the throat way up. Make the short side radius, as big of a radius as possible. Creating a perfectly rectangular port is the most common way, and what makes everyone feel safe and cozy, but its best to cut the port into a trapezoid shape...enlarge the width of the floor, and leave the width of the roof alone, although its best to cut the roof into a D shape. In the end, its like a tall upside down triangle with the tip cut off. This increases flow in all the right places and prevents lost velocity. Squeezing the air in the area where the width is tight, pushes the air down to the area where velocity and volume are usually low. Now you end up with higher average volume and velocity, throughout the entire runner...not just the roof and the side wall with the positive curve. GM figured this out with the LS1. Look at those runners.
So to answer your question, find a good porter. One who does heads for lots of winning race cars, not some dude who you heard does good work. Ive seen those jobs and they're really nasty. Many times done by someone who never ever worked as an automotive machinist, and it shows in the quality and tolerances. I would keep the stock valve sizes, do all the right valve and seat work to get the most low lift flow, and do all that other work to increase flow at the higher lifets without hurting velocity. Only thing about doing it this way, is that you have to modify the gaskets, and port the intake runners on the intake manifold to match.
