Slant six head
Did you use vavle quides if so what matiral because i see iron valve quides for sale on rock auto but i have seen people talk about bronze valve guides being the way to go.
Yes, keep the stock sized valves and install new valve guides and positive valve seals.
I would definitely let the machine shop doing the work source the valve guides and positive valve seals.
Have the MS quote both bronze and iron guides and then see if the price difference is worth it to you. Billions of motors have used iron guides for billions of miles.
I would also let the MS quote the valves, and the springs and the keepers. they can likely buy as cheaply as you can.
Your existing retainers should be good for re use. If you are putting in new valves I would put in new keepers.
Ask the shop to itemize all the parts on the estimate. You need to already have a good idea of what you can buy the parts for. Then you can make a comparison of the MS part pricing to what you can buy the parts for. A good shop may put a small over charge on the ordered parts to cover the time they have invested in the ordering, stocking,,, but if they are laying it to you, that is a good reason to find another MS to work with.
About the only time I bring parts to a machine shop is when I am using valves / springs / retainers or other components that are not stock but that is what I want.