UUUHHH... Make sure your heads don't have any defects or cracks before you spend all that time on them. ( Don't ask how I know)

Yeah to what he says! Not sure exactly what heads you have but what Kennyo says is especially true if they're the GM cast iron rectangle port heads. They have a tendency to crack between the seats. USED GM rectangle port heads are becoming easier to get because of all the other head casting options available from aftermarket manufacture's these days, but there is always the random risk of the GM rect heads cracking. I have seen it many times in Marine applications since the early 1980's. Some guys spend a lot of time/money on porting/preping the GM rects only to have them crack....but some guys are very lucky. It's a roll of the dice. If your wallet allows it try and buy an aftermarket casting like an Iron Eagle 308cc or something similar.

Ive seen and had the cracked head in the seat area before, They drill it out and put some kind of plug in it. I havent had a prob , But I guess it depends how big it is?

The "feel" of the exhaust port short radius surface is more reliable than visual do to location. Just rub your finger across it in the direction of the air flow and you will be surprised how sensitive that can be it finding non uniform spots to blend.

Thanks!!