This is the data I have for a 4.6, I DNK what a v6 takes.
MOTORCRAFT
AWSF-32C / stock heat range - copper plug
AWSF-22C / 1 Heat range colder - copper plug
AWSFA-12C / 2 Heat range cooler - copper plug
AGSF-32C/ Recommended for FRPP heads(not sure on heat range)
NGK
TR-55/ 3951/ Stock Heat Range
TR-6/ 4177/ 1 Step Colder
BP7EFS (PN - 3526) BR7EF (PN - 3346) two steps
R5724-8 (PN - 7317) two and a half steps
R5724-9 (PN - 7891) three steps
R5724-10 (PN - 7993) four steps
AUTOLITE
764s / Stock
103s / 1 heat range cooler < this is what I'm putting in. Autolite makes the Motorcraft brand.
AR94 / 2 heat range cooler
AR93 / 3 heat range cooler
AR92 / 4 heat range cooler
CHAMPION
YS-59C / -3 or -4 below stock heat range
DENSO
IT16 / stock heat
IT20 / -1 heat range
IT22 / -2 heat range
IT24 / -3 heat range
Small update
Motorcraft sparkplugs have changed part #'s and some are now only aval in a full thread design, new part #'s are as follows......
SP-447 replaces the AWSF-32-C & AGSF-32-C, Full Thread design-stock heat range in a copper core
SP-505 replaces the AWSF-22-C & AGSF-22-C, Full Thread design-1 step down from stock, also copper core
SP-494 replaces the AWSFA-12-C still a 1/2 thread design-2 steps down from stock, copper core