2 Questions

First, I'm installing a Trans Temp gauge in my '99 Yukon (Autometer.) I need to drill a hole in the transmission pan for the temp probe. Any ideas/suggestions where (side, bottom, right/left side, front/back?) Can I use an aftermarket extra capacity pan? Are 4L60 and 700R4 pans the same?
Second, has anyone installed OnStar in a non-OnStar vehicle like mine?
Thanks in advance!