I'm going through this with my 340 right now also. I used a magnetic base dial indicator to find the distance the pistons are out of the block. I used a 1/2" piece of keystock and layed flat across the cylinder surface then set the dial indicator at zero on top of the keystock and rolled the engine over and measured the total height out of the block. I would assume any solid straight edge would work. Mine were right on the money at .018 I had a set of magnum gaskets that mic'ed out at .065 and after tourqed to specs they compressed to .055 using feeler gauges to check the distance between the block and head. I also had a set of the felpro .053 gaskets but I liked the the thicker ones to be safe. BTW I'm also using magnum heads on the 340 similar to your's