Depending on which extractors you get, you still might have clearance issues with right angle plug leads - depending on how deep they are.
I can't recall which pacemakers I have atm, but I had problems with leads touching them from the heads (have gt-40p's as well). The solution seems to be reversing the leads(!) Seems to work ok, and the dizzy ends of the leads are more compact and clear the exhaust nicely.
Of course it took me about 3 melted leads to work this out
And yes, the stock extractors will not fit gt-40p's (not without "modifications" to the extractors anyway (read oxy torch + hammer).
If you are doing the work yourself, give yourself lots of time and make sure you have all the tools at the ready (including psp puller and socket drive torx bit for the serpentine tensioner(T-25 I *think*)).
Also, while I'm not sure of the pricing, it might be worth investigating getting the inlet manifolds flowed a little while they are off. It won't be as nice as new/better ones - but it should be an improvement (after the head change the inlet manifolds will probably be the big bottle neck).
It sounds like we are basically going down the same line, next up for me is going to be a new maf to replace the tiny 55mm EL one . . .
Apart from herrods, you could also try City Ford in Melbourne,and Mick Webb of SVOMotorsport - it'll pay you to ring around . . .