What to tell the machine shop

I have talked to 3 different machine shops in the area, and I am being quoted $850 labor plus parts. Since my knowledge of valve lengths, cams etc is limited, I am not able to answer all of the machine shops questions.

So here is where I am at. I want to have a 1968-74 head machined to accept larger valves, springs, some porting, milling, and change over to street performance cam.

My engine is a stock build with 2000 miles on it, and I do not want to take the head off, so I can drive it until I have everything to perform the job.

I do not want to have any clearance issue, or notch pistons, etc.

Should I buy a complete kit with valves, cam, pushrods, etc, with a known combo that does not require any guesswork? The shop is asking me if I need to use a different size pushrod.

Or, is it just a bad idea to have the head done ahead of time?

This is a slant 6