I can tell you the yardstick I use, which has served me well;
In neutral,with a vacuum gauge plumbed to the intake manifold, slowly rev the engine up until the vacuum plateaus and does not rise any further, or at least drastically slows down in the rising. The lowest rpm on the plateau is the rpm at which the pistons are no longer pushing air charge back up into the intake; so all that has been inducted, is now being compressed. That will be your lowest EFFICIENT cruising rpm. You can cruise at a lower rpm but the intake is gonna get messy. You can cruise a lil higher and not lose too much.
To find your cruise timing at that rpm; (edit; minimum cruising rpm), just keep tugging on the timing, while simultaneously reducing the rpm back down to the minimum cruising rpm, until the engine no longer builds rpm. Edit; I use the fast idle cam to set the minimum cruising rpm.So for example; you add 3 degrees timing and the rpm goes up say 200rpm. You back it down the 200 and add 3 more degrees. This time it might only rise 100rpm. So you back it down, and add 3 more and this time it goes up only barely, so you take the last 3 degrees out and call it done. End Edit.
Badaboom! Now put the timing light on it, record the number,return the engine to idle and retard it back to the "Before-test" numbers.
Don't be surprised to see mid-fifties for timing; or more.
and do be surprised to see rpms less than 1800 with typical street cams that have Ica's between say 58 and 66 degrees.I'm gonna guess plus something like 80rpm per degree of Ica; so a 64* Ica would need ABOUT 2280rpm minimum.
Now you have the test-drive numbers; all you gotta do is gear the car to achieve whatever speed you wanna cruise at, and blast up the timing, and start leaning it out, until it won't maintain the speed, or starts acting funny, or burning stuff up; then fatten it back up a lil. And of course a lil fine tuning.That's the best she's going to do at that speed; if you want higher numbers, yur gonna have to either; slow down and regear, or lose weight, or get a slipperier body-style..
There are just two hiccups;
#1) how do you give your small-cammed engine, 56* of cruise timing at 1800 rpm? Good luck with that................. I think that it cannot be done with any factory distributor ever produced, which if you really twist up your timing, might get 50*, but more typically with that small cam, hi-compression combo, you might get low 30s* which Ima thinking is 18*@1800mechanical, plus , 12 to 16 in the Vcan.
And hiccup#2) is; you automatic transmission guys who maybe can't even cruise at 1800, without a lotta TC slip. But if you have a loc-up, this applies equally to you, except with a hi-stall, for some reason you guys like to run 30 degrees at 2400 or something equally wild,and you can,so you might have an easier time getting the 50, but you still might not get to what the engine truly wants.
This is where cylinder pressure really shines; and why modern EFI cars are again being built with high compression ratios, and computer controlled timing..
I solved the problem with a dash-mounted, dial-back, timing box. Mine has a range of 15 degrees. So I can optimize my power timing, and optimize my PartThrottle timing, and my Vcan assist, and once at cruising rpm, dial in whatever more she wants.
The hard part is remembering to dial it back out, when I slow down for a town,lol..
But that's another reason that 367 is wearing aluminum heads; they put up with a lottachit.