What sparkplugs to use (brand, model)? I have 383 with comresion ratio 9.8 and max RPM 6000, naturaly aspirated, standard Crusader ignition (Prestolite distributor) .
We have NGK, Bosch, AC and Champion available here in Estonia.

AC MR43 or 44T or NGK equivilant. Try a set of each go out and run the thing hard and shut it off, pull a couple plugs from each bank and take a look. Your igntion may start to fall off before 6k though.

cig 3 is on the money, ac delco mr43t or 44 / or ngk br6fs what ever you do stay away from champion spark plugs