What is the most acurate method of locating and drilling the transom for tail pipes.

What system are you installing? Different methods for different exhaust. Is it a manifold with separate riser? Is it a header with a flange or a pinch clamp? Do you have the pipes or are you going to make them to the holes you drill?

We are installing CMI Big Tubes - have not yet cut holes in transom, nor ordered tailpipes. Tailpipes will be straight. Interestingly, we have found the collector angle varies between 1 and 3.5 degrees across the four collectors, and distance from rear of the block to transom is 42 inches. So, as a result of angle variation, over 42 inches is a variance at the transom of 2.2 inches. These are new Big Tubes with the flex flare collar which will allow for some adjustment. Just looking for insight.

Since you are buying tail pieces from cmi I would ask for their measure sheets. Then look where you would like to place holes visually and make decision. They will make them to your holes. Pay attention to the measurements that are important to them.

Thr tail pipes are to be straight. However, perhaps there is a proven method of locating the holes.

Thanks for your help.

Call up CMI (920-720-4225), ask for David Rank. He is a good man, and can help you with this. I do believe that flex flare collectors have 3 degrees of adjustment. He can determine the proper tail drop and hole location for you. Better to ask then cut, and patch.
Bawana