Hi Eric, assuming it's a new block (as delivered from G.M.) you will need to have it finish honed with a plate. You will also need to have EVERY other dimension checked before you start laying out the program.
Most will need decking as well. The ones I have here are already "square" but they are slightly high from the 9.800" number. I have built some with the decks "as-is" with no issue! Depends on the platform and the customer.
I have about 4 of these (MK-VI style) here now based on the 4.250" bore, and I have a bunch of the BB 4.500" Bow-Ties (MK-IV) as well. Most are all in their original boxes from G.M. Some have been started on their builds.
I have no "502's" handy but on the 4.250" bores G.M. leaves the bores (unfinished) at 4.244" (nominal), and the 4.500" designated bores start at 4.495" "as-delivered".
Most of our builds end up at .001" over the "standard", e.g., 4.251", 4.501", etc.
We haven't found it "mandatory" to correct any of the main bores, cam holes, or even the lifter bores, they are normally fine as-shipped. This is our own findings. I use a number of these castings, mostly the 4.500", MK-IV style, S/D though.
I do try to avoid any other brands and stay with G.M. but occasionally my customer's will request the Dart line.
(Add) I really don't care for the 1-piece rear main seal setup, it creates other issues for us! I prefer the MK-IV deal when possible. I have used a number of rear seal "adapters" to convert the blocks with excellent results.
Thanks, Gary in N.Y.
P.S. You really need about .005" minimum material to remove (we prefer to see .006") to get a good base finish on the end bore size. We've had no issues to date with ring seal!