A popular choice for nitrous plugs are NGK TR6s. They're one step colder copper cored plugs. You can order them next day from Advance Auto Parts for like $15.
Motorcraft AWSFA22C gapped at .032 If the dealership doesn't have in stock they can order. They are cheap, reliable, and commonly used in blown or juiced applications for 99+ cars.
This is one heat range colder than stock and should serve your needs just fine.
autolite- 103's, denso- IT20 but I wouldn't reccomend the Denso plugs.
the other parts listed are all 1 step colder. Wall Mart has the autolites sometimes and are about $1.00 each. Advance also carries autolite. Ford usually has the awsf22c( make sure not to get awsf22p, those are platinum) .
I found this in a thread somewhere, maybe this will help.
MOTORCRAFT
PART# / HEAT RANGE
AWSF-32C / stock heat range - cooper plug
AWSF-22C / 1 Heat range colder - cooper plug
AWSF-12C / 2 Heat range cooler - cooper plug
AGSF-32C/ Recommended for FRPP heads(not sure on heat range)
NGK
PART# / HEAT RANGE
TR-55/ 3951/ Stock Heat Range
TR-6/ 4177/ 1 Step Colder
FOR 99+ CARS w/ .708 thread depth
PART# / HEAT RANGE
BP7EFS (PN - 3526) two steps
R5724-8 (PN - 7317) two and a half steps
R5724-9 (PN - 7891) three steps
R5724-10 (PN - 7993) four steps
AUTOLITE
PART# / HEAT RANGE
764s / Stock
103s / 1 heat range cooler
AR94 / 2 heat range cooler
AR93 / 3 heat range cooler
AR92 / 4 heat range cooler
CHAMPION
PART# / HEAT RANGE
YS-59C / -3 or -4 below stock heat range
DENSO
PART# / HEAT RANGE
IT16 / stock heat
IT20 / -1 heat range
IT22 / -2 heat range
IT24 / -3 heat range
I am using the NOSzle system with the 125hp shot on my 2001 GT. I decided to go with the Autolight racing AR94, which is two ranges colder. So far I have been extremely pleased with there performance. The AR94 is a copper core plug with the electrode cut back. I have my gapped at .040 and everything seems to be running great.