Here goes:

Install the blocks on the firewall. Put a piece of masking tape about 6" long from the block rearwards. Using a ruler draw a straight line right on the tape, from the center of the block back a certain length, say 5" and mark the tape with an X. Do this for all the blocks.

Put the cowl in place, aligning it to the crankshaft, etc. Tape the cowl in place. Use your ruler to measure along the line you drew on each piece of tape that certain amount (5", for example) forward to the blocks. The end of your ruler should be at the center of the mounting block.

Drill pilot holes and mount your cowl using whatever method you chose