I'm looking at purchasing a set of 2.0 stock heads that have 8 mm exhaust valve guides.

My understanding is intake guides are 8 mm and exhaust are 9 mm.

I assumed they welded the heads to drill out the bore for 8 mm guides, but is that okay?

Also if they used intake valve guides all around is the length difference significant?

Pretty sure the exhaust valve guide has a larger O.D. under 16mm and the inlets are over 13mm,they don't interchange.
Make sure the heads you are buying have new exhaust valves and guides...minimum.
I never reuse exhaust valves its not worth the risk.