1973 455 Stage 1: now 8.5:1 compression, how much higher possible for 2018 Premium fuel?

Tough to say about the condition until after the fact, but again I'm saying the flow testing using cfm takes a back seat to the actual port sizing. Porters aren't only looking at the cfm.

Lapping? irrelevant, that's just for the last few tenths after it already seals and to see the contact pattern.

An example is a set of fancy alum. race heads already serving 5 years duty that I ported for serious hp upgrade.
The engine had a big roller cam that beats things up and was run into valve float to the point that the retainers and valves were junked.
The seats actually looked OK visually but flowing them many steps along the way determined that a light bump with a grinding stone added back 12-15 cfm...nearly an automatic 25-35 hp and helping the port's airspeed at every lift point.
(not important for yours, just to illustrate)
Normally people have things freshened up to ascertain condition prior to sale and flow them in best possible condition.
This doesn't automatically mean the heads are the best now for any application, it depends on what the porter's original intentions were.

If you are asking to determine if you need to pay for freshening, then flowing, then further work, then don't bother.
Let your shop do what they do.
That's the potential micromanaging it appears like, although you are asking just for informational.

 

The angles and diameter of the valve job either by the rebuilder not having the same stuff as the porter or how either decides to do things can easily affect 30 hp, so there's a certain amount of irrelevance to cfm testing.
It's a rough way of looking at things.

Whet you 'could' do is compare other Buick builds having your similar flow sheets to see how it acts from wild to mild based on the overall combo.
Your builder's already now what to do, and IF the heads are more than they expected could advise you on their proposed solutions. One being to simply make the engine bigger as it solves many evils.
Many of your concerns are actually already answered in previous comments.

 

The heads are in the hands of my rebuilder now, and I have only asked him to bench test for flow before he does any work on them, and before we decide if we really use them, or use my old heads. Is there anything else you suggest I ask or tell him?

My original Stage 1 heads are 100% original with 160,000 of hard miles on them, so they will need a lot more work than these flowed ones. These flowed ones, I believe, were not used much by Dave B, then retired in favor of aluminum heads. This is according to the original head guy, Steve, who actually did the flow job on these heads in the first place. Tom had him looking at them again, and that is where I picked-up the heads after I paid Tom. So, I asked A LOT of questions of Steve. Basically, he doesn't remember much from the past, only what he sees now: "Very big job of flowing, so big that some of the head bolt holes had to be sleeved", "valve and bronze guides' clearances are still excellent", and "no need for a valve job on the valves and seats, just some lapping".

I am trying to be sensitive to 8ad's suggestion, and let this rebuilder do his work and make his suggestions, without demanding too much. Even though I am getting all this input from the guys of V8Forum and from Steve, the head guy. The rebuilder seems very receptive to my comments, particularly since I always tell him I am just learning, so I trust his opinions more than my own, and just passing on to him what all these other hobbiests and experts are trying to teach me.

 

Not having to do much to these might be a huge boon to the project

 

Actually on the engine, I removed them from engine about a year ago, engine actually running about 25yrs ago, was in the process of just getting in checked over, new springs/retainers, flowed and then sell when all this occurred....

Steve owns BPE....his work goes from basic to fuel engines....http://www.bpeheads.com/index.html

 

The timing was perfect for me, if they work out. And I think they will. I have "High Hopes".

8ad has me thinking about a roller cam, with high lift but narrow duration. I am trying to resist, but it sounds like a wow-wee solution, except that I still have my exhaust manifold and 2-1/4 exhaust that I want to stay with.

Nope, I am sticking with my plan, using the cam I have, too. It is up to the matching of port dimensions (for the intake and exhaust manifolds), and finally the custom pistons to make everything come out right.

 

If you're going to get customizable pistons anyway, you can get a set of these;

https://www.ebay.com/itm/CXRacing-H...ash=item2578e03cab:g:cDEAAMXQVERS4Kg7&vxp=mtr

Order the pistons with the wristpin size for the rods, the compression distance needed to zero deck them and have the crank offset ground to a 4.055" stroke, with a 4.350" bore will give you 482 cubes! Should only cost around an extra $200 for the offset grind, the rods are .012" wider than the BBB rods so either the rod journals need to be widened .012" per side so .024" or the .012" needs to be removed from the rod's big end to make them fit.

The above is the budget 482 recipe. More cubes = more low end torque, if those heads have slightly lazy ports from a lot of porting, the extra cubes will help wake them up.

 

Make the engine bigger and you'll be able to keep a smooth-ish acting idle and low speed with more cam than the one being considered.
It tames the behavior when the cubes go up.
A 214* @ .050" cam in a 350 sbc is sort of rumpity but it's nothing to a 455 or 500.
A 230* @ .050" begins to rumble in that size engine.
Personally if there is a budget choice between a roller cam OR cubic inches, I'd choose the cubes no matter what.
(Not that this is a choice for you)