Depends on whether you want a warmer plug or colder plug. Stock heads take a 1/2" reach plug (barrel length with the threads), and the GN-1 heads take a 3/4" reach.
Stock head GN-1 head
Autolite 44 (warmer) Autolite 104 (same as 44)
43 (colder) 103 (same as 43)
NGK UR5 (warmer) NGK TR5 (same as UR5)
NGK UR6 (colder) NGK TR6 (same as UR6)
The warmer plug is closer to the normal heat range. I run the NGK TR6 in my car, but take your pick. I like NGK better and are about the same price. What I mean by "same as" is the heat range is the same, but with different reach.
Some other people might recommend something else, so you might want to try others to see what you like best.
HTH
