I have a 600 hp carbed 502 with alum afr heads just tested the a/f ratio which is 13.7:1 and my comp ratio is only about 9.8:1.After a short time #5 and#6 plugs are fouled and 7&8 are starting also to turn black. I am using NGK v power R5671A-9 plugs with a .035 gap and Crane Fireball ignition box these plugs are the deep thread design and are wondering what other plugs and gap I may want to try. Thanks.

Is that 13.7 at idle or throughout the rpm?

open the plug gap to .045 and see how it does. dont know about the crane ignition but msd reads to open the plug gap +.010..

I would check the a/f meter, check for arcing in the cap. If you really are at 13.7 and plugs are fouling, you must have a significant ignition problem. I've run my plugs at .035" with an MSD 6M with a Blaster 2 coil and I have idled for 4 hours taking long river cruises with no fouling and I am running NGK 5674-8's. I'm running 9:1 compression with a pretty big cam.

The 9's you are running are WAY to cold. You should be running 7's (5671A-7). An afr of 13.7 is fine at idle and part throttle (low load) but at higher loads and wot, you need to shoot for about 13, or possibly high 12's.
I use the 9's in engines making almost 1200 hp and beyond. They are very cold for your setup. That's why they are fouling.
Eddie

thanks for the comments... I have to recheck the rpms vs the a/f ratio numbers.I think I'll try the -7 plugs per Youngs perf but what gap with the crane box? Also what is the effect of colder vs hotter plugs/ Thanks.

run them .035-.038, the heat range of the plug in a sense acts like advancing the timing, with the 9s you'd have to run quite a bit of timing to get the heat in the chamber during combustion but foul them at idle when advance is low, hotter plugs you can run a few degrees less timing and still have the heat at lower loads and advance, hope i explained that well enough

Too cold of a plug. Like Eddie said, go with the -7's. I run the 7's with 10.25:1 compression in a carbed 598ci and never foul the plugs at idle. When i had -8's in it, they wouldnt foul but would need a little bump every few minutes to keep from loading up.

I run the NGK R5671A-7's in my 800HP N/A engines gapped @ .045. They work just fine.