exhaust manifold studs

Can anybody tell me what size I need for the studs for e. m. to Y-pipe? length and thread size?

That information is available in a Dorman catalog. A good auto parts store, such as NAPA, should have that catalog. A GM dealership parts department might only be able to give you the GM part number and not the thread specifications.

Here's one other tidbit.
I picked up a plastic template at home depot recently while looking for other things. It's called a "gage-it". It measures sae and metric bolt and screw sizes and thread pitch. I used it recently to replace the nuts on my y-pipe to manifold bolts.