being your only pushing 7 psi, stock iridium plugs would work just fine.

can you see a name on the old plugs or a #? if they are platinum that could also cause misfires. these engines dont like the plat plugs one bit.

you could also use copper core plugs, at the stock heat range. they are cheap at 2 bucks a plug but only last 10 to 15,000 miles.