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Ive read about resistor plugs, and it all looks good in theory. But in more recent times Ive recalled that there is at least 2700ohms resistance in a typical lead, so im wondering how much of a difference 4ohms more in the spark plug would make. Any ideas radio engineers?

Got an answer from a radio engineer mate:

The 'resistor plugs' may help, as I think that rather than just being a
resistor the 4 ohms could be via a wound inductor within the plug. This
inductor would attenuate RF type noise whilst appearing short circuit to
the voltage from the coil. The spark gap transmitter was the original
method of getting signals across the Atlantic Ocean. The 'resistor plug'
could stop RF radiating to the leads, in my opinion, from that nasty spark
gap within the cylinder head.

From NGK site:

NGK automotive resistor spark plugs contain a ceramic monolithic resistor. The resistor filters out excessive electrical noise generated by the ignition system. The most effective place to situate a resistor in the high tension circuit is as close to the spark plug as possible making the spark plug an ideal place to house the resistor.

In nearly all cases - apart from some very old low output ignition systems - resistor spark plugs can be used in place of the non resistor versions with no detrimental effect on engine performance, power output or vehicle emissions.

So much for the inductor theory.. looks like it just provides isolation.
Resistance values seem to be in the order of 5k ohms, which makes a bit more sense.

well at the end of the day, in stock plugs or race series plugs there is resistor and non-resistor options so im going to be doing for resistor!

i've spoken to my speedway mate and the difference in the plugs he runs compared to stock for the motor is pretty much different heat range and also less projected tip so i'll aim for the non projected ones to start off with and see how i go i guess....

non-projected help with knock as well they arnt way out in the heat

The 12 port that I refer to is a blue/black head, not a red 9 port converted to 12 port.

bit of an update on things, got the head at me mates workshop, last night we roughly cut the larger valve seats then used the cutter to throat the area under the valve seat, it really worked out pretty well, only leaving some minor porting to be done to have it all flow in together, so that will be todays task to get the porting done and then check valve seat pressures then assemble!

a funny side note to all this, a few years ago when i last freshened up the head i had to get hardened exhaust valve seats installed because the original seats where almost 2mm recessed into the head on some valves, so anyways, with the larger valves we've almost machined out the entire hardened seat!

very productive day yesterday, got all the head mods done, only issue now is that with my new cam i need different springs so got to order all that stuff before i can put the head together.
But for now i dug up some old pics of the head and got some comparison photos of how it is now.

so this is how the head was before any changes
and this is how it is now
so inlet valve sizes have gone from 1.6" up to 1.75" and exhaust from 1.4" to 1.55"

you can also seen in this pic below the valve stem areas have a bit of meat left around them
and in this one alot of that has been removed to hopefully make for a smoother area for the air/fuel to flow past
So you can also see in the last picture that the area bewteen the valve and edge of the chamber has been machined, we have taken it out to the edge of the gasket to de-shroud the valve as much as possible, there was probably 2mm on some and a bit less on others, shows up that the casting isn't all even but its pretty damn close really.

so todays task is to re-cc the head and see how much its changed with all the mods (less cc's from the valves being further into the chamber and the head being decked - more cc's from the de-shrouding etc) i'm hoping its slightly less cc's if anything but want it to be pretty much the same so i don't need to touch the block other than a light skim to true everything up.

Just a novices observation, are the bigger valves going to suffer from to much shrouding and effectively negating there size increase?.

It is hard to see in the images if there is sufficient space around the edge of the valve to allow flow as soon as it moves of the seat.

i don't think so, what we did to de-shroud them seemed to be about the same distance between the side of the chamber and the valve when compared to a stock port before de-shrouding it so im fairly confident it wont be a bad thing .... add to that the camshaft i'm putting in it is going to have alot more duration and lift so it will make up for it if there is an issue

im going from a .050 duration of 218 degrees to in238/ex248 degrees and lift of .487 to in.517/ex.533 .... and add to all that im going from a hydraulic cam to a solid cam so it should be a reasonable upgrade ... also the head is pretty much on par with a yella terra now, and if anything i reckon more ported than a YT head is from the box!