Chevy c10 spark plugs..

[Gav2606]

Hi all. Ive got 68 c10 with a mid 70's 400ci chevy motor. It was already fitted, it has edelbrook performer heads and inlet fitted. Im looking to buy spark plugs and every 400 ci engine listed i can find seems to take shorty reach plugs. The one fitted are autolite 3926 which have .750 reach..could this be because of the heads being changed or is there a year they where fitted. Im in the uk so limited information thanks in advance