OP did say one key thing
"stock convertor";
From which I can also infer stock rear gears, which are likely 2.76s..
Therefore, like
@Toolman said, the cylinder pressure could be way in the basement, Plus the big Airgap on this little combo, is IMO a totally bad idea. The Ag is a great intake, but way too much for any 273. So all in all it's a bad IMO combo, if not just an unknown one.
A compression test is paramount to figuring this out.
But it gets worse;
The stock convertor and the 2.76s (if I guessed rights) conspire to make this a poor runner, with any cam at all that has a later closing intake angle than stock.
As it stands, with the information given, the stock 273 cam is already the right cam.
With a true 9.0 Scr, (closed chamber heads and a total cylinder volume of 70cc) the Wallace predicts a pressure around 158psi, with the Hydraulic 273cam (sorry, I do not have the specs for the solid). This is pretty good.
But
with the 318 open-chamber heads the total chamber goes to ~83cc, and the math indicates a compression ratio of
just 7.7, which with the
48* Ica of the stock HFT 1969 cam, drops the Dcr into the basement at 6.8, and the pressure to ~129psi.
Going from 158 to 129, is a very serious loss of pressure, and so, with a bigger cam, is a recipe for serious disappointment.
For instance; a 262/268/110 cam might have an Ica of 57* which would drop the cylinder pressure to 120psi and the bottom-end is now into slanty territory, having a V/P of just 81, compared to the 9.0 stock cam at 114. That represents a loss in low-rpm performance of ~29%....... see what I mean?
Saddle that with 2.76 gears and a 2000 stall that is now just 1800 by way of the mushy V/P, and I can see the tears running deep.
if the 273 was mine:
the First thing I would do, is to swap out that large port /large plenum Airgap for a small-runner, small plenum intake. and if the heads are open-chambers, they gotta go.
Second would be a higher than "stock stall" convertor, and dual exhaust.
Third would be to install as much rear gear as I dare.
Fourth is a compression test to see where it's at.
Now; with the hi-stall and gears, cylinder pressure is not as critical as if the car had a clutch, so, now back to a true 9/1 Scr, I can entertain a later-closing intake, but if I install a cam, then I'm also getting rid of the 273 log exhaust manifolds, in favor of small-diameter tuned pipes, aka headers ...
That is how I would do it.
Notes;
All numbers quoted are for the 68 up HFT. The Solid number may be a lil higher but it won't be by much.
In the calcs I used 57cc for the stock closed chamber heads, and 70cc for the open-chamber 318s.
I assumed a stock compression ratio of 9.0 for the 67-273.
The overlap on a 262/268/110 comes to about 44*. If you don't take advantage of that with headers, then you might as well not install the cam, because the 273 logs will kill that overlap, robbing you of a good amount of power thru-out the rev range.
To the OP, a cam makes power by moving the engines operating rpm ever higher. In so doing, it trades away low-rpm power. The percentage of loss/gain is very similar. If you gain 5% power at say 4500, yur gonna lose 5% off the bottom. To get the bottom back, you gotta increase the stall. Keep doing that and eventually the stall gets to be too high to be comfortable around town, and hiway fuel-economy takes a huge hit.
AND
If you run hi-way gears, it puts that ever-increasing peak-power, at ever-increasing Roadspeed, keeping your engine struggling for the vast majority of time.
Each cam size moves the power-peak up approximately 200 rpm.
your current cam peaks around 4400. which with
2.73s in First gear, is ~53mph. Going from your 240 degree cam to a 262 cam is 22* or about 3 cam sizes, or about 600rpm. With your new power-peak at 5000, this is now 60 mph. To get it back to 53 mph would take
3.09 rear gears. But you don't really want to wait that long do you? No! You want the power at stall rpm,
With the current 2000stall, this is not much power. With a later-closing intake , it will be even worse.
Lets say your engine makes 200 ftlbs at 2000 rpm, which would be 76hp.
Lets say you install a 2800 stall and your engine is up to 220ftlbs; this is now 117hp. From 79hp to 117hp is 40hp, which is a much bigger gain than the 4bbl is at these rpms, much bigger; and far more power than a bigger cam which, with no other changes, at these rpms, will LOOSE power .
You see how the stall works? It's a wonderful tool if you don't have a clutch.
