is no.1 cyl and no.8cyl at TDC at the same time?
No, with a flat crankshaft in a 90 deg V8 block, the two pistons that are at TDC at the same time are always in the same bank. Are you clear on how the cylinders are numbered? Another way to see this is to look at the firing order of 1-5-3-7-4-8-2-6 with occurs over 720 deg (2 revolutions) of crankshaft rotation. The pistons that are at TDC at the same time are separated by 360 deg of crankshaft rotation - i.e.: 1 and 4 are at TDC at the same time 5 and 8... 3 and 2... 7 and 6...
For the 5-8 bank, you would rotate the crankshaft 90 degrees in the direction of normal engine rotation (from what you were calling TDC for cyl #1 where the #1 intake valve is just opening) to the PM5-8 mark (where the #5 intake valve should just be opening), reset the degree wheel on the crankshaft to correspond to TDC for cylinder #5 and then measure the cam lobe motions for cyl #5. Your flywheel should have a PM1-4 mark = TDC for cylinder #1 (and cyl #4) and a PM5-8 mark = TDC for cyl #5 (and cyl #8).
