help drill bit size to tap hole correctly

[brans72]

I am installing a Auto Kraft oil pan on my ls1 block but there is a extra hole on the oil pan itself. I need to tap the block but I am not sure what size bit to drill with to tap hole to 8x1.25 to match other holes. I have talked with a few people who say it must be tapped or will leaki and we know we do not want that so what size drill bit do I use???? You can not tap with oil pan on due to pan block you froma straight angle to drill/tap. So please help me out if you can guys.