I have an 86 Firebird with a 350, Edelbrock Performer intake and a Holley 670.
I know I need an off set base air cleaner to clear the HEI.
I was looking at a 14" by 3" tall, just wondering if a flat base will clear the hood or if I need a drop base?

Why do you think you need an offset base? I've always used typical air cleaners and they've always cleared the HEI. Just barely, but they clear.

I have always been told if you have a large cap HEI, you need the offset air cleaner to get decent clearance.

you will need to use a stock air cleaner housing if you have a flat hood and don't want to cut a hole in it.