If you are interested in making more power, the heads are where the power is at. You can flow just as much air through cast iron, as you can aluminum. It requires port work. The difference is aluminum is much easier (softer) to machine. The other advantage of aluminum is heat conductivity. There is less hot spots in the combustion chamber, so you can run a bit more compression. I digressed. The point, work on the ports.
If head has pressed in studs, pull them, tap them, and screw in studs.
I have read enough to believe the ZDDP and flat tappet cam thing is real enough to matter. It's your money.
I would spend the money for a roller lifter cam set up, if I cared about power. The roller lifters allow for much steeper ramps, which gets the valve open sooner and holds it open longer than a flat tappet with the same duration. If you cannot afford it or don't care about power, then the flat tappets will work just fine, but I would add STP or some other ZDDP to the
oil.