I'm coming out of retirement to respond to _this_ post!
Since the heads relocate the spark plug placement, the plugs actually have a 1” reach vs. the usual plug with a ¾” reach. or in mm, a 25mm or 26.4mm plug will work.
Here are the plug choices:
For SC cars: NGK LFR 7AIX - 2 ranges colder than stock. Around $7 each
For you, the NGK will be a perfect plug.
To buy spark plugs, the best price around is usually
www.rockauto.com or sparkplugs.com . you can also call sparkplugs.com on the phone, place and order, and tell them you have a company. if you make up a company name they don’t actually check or anything but they will give you 20% off or more on your plugs. That is actually pretty significant when you buy 20 $12/ea plugs.
So go change those plugs, get the car COMPLETELY re-tuned, and then let’s hear about it in a NEW thread!
Joseph