think there is a few that are different lengths iirc the 12 has shorter ones in some spots

 

Yeah, you need to replace the 6 on the passengers side of the block with the shorter length ones for the 12v. That is a block thing, not the head though. You can probably just call ARP up and have them send you 6 for a reasonable price. Is the 24v head already cut for fire rings?
Sean

 

I think it was Ian/ICSmoke that did this to his motor if I remember right.