Ok I'm confused;
Why would you run that 296* cam at 1500rpm for so many hours?
What were you thinking?
You wanna bet your engine oil smells like gas?
-------------------------------------------------------
Here's what I would do;
A) at your cruise-rpm of 1500 rpm; check your ignition timing and your manifold vacuum, then, disconnect the Vacuum advance and see if it changes. If it does, make a note of the mechanical timing by itself.
Next;
B) Run the rpm up to 1500 rpm, then fix it there for the rest of this test.
Reinstall the vacuum advance, if the rpm goes up, slow it back to 1500. Then
advance the timing; and if the rpm goes up, slow it back to 1500.
Repeat until additional timing does not cause more rpm.
C) Read the timing.
Whatever you read for timing, that is what your engine wants, at this rpm, to put the pressure peak in the right spot. It could by 20/25 or as high as 45/50, IDK BBs at 1500.
and, read the manifold vacuum, at this setting, then, compare what you see, to what you had in A) above.
If you cannot supply your engine what it wants, or close to it, then don't cruise there.
Here's what you need to know;
Your engine has a point in it its power-cycle, in the which the pistons will be in the optimum position to transfer the energy of the expanding gasses to the crank. All your attempts at adjusting the timing, is only adjusting the beginning of the burn time to make the peak pressure occur at that optimum point, which is around, 25 to 28 degrees AFTER TDC Compression.
If the pressure comes too early, it may cause problems, as the rod hammers into the crank, before it's ready.
If the pressure comes too late, then optimum power cannot be achieved as the piston is already running away from the expanding gasses.
This is especially crucial while cruising at steady rpm. Too early or too late will cost you fuel economy; and the loss of manifold vacuum causes other problems.
In your case, with that big cam, the pistons are pushing just inducted A/F charge, back up into the intake plenum as they round the bottom of the stroke and the valves won't be close to closing until the piston is over 72* past BDC, on it's way back up on the compression stroke now! This is why your manifold vacuum is so low at idle.
This is further complicated by the cam having some 80* of overlap, during which time, the headers are desperately trying to pull AF charge straight across the pistons and out the pipes. The slower the rpm, the more successful they will be.
1500 rpm is a terrible rpm to cruise that combo at, only made worse by lack of Ignition advance, which doesn't help the low manifold vacuum.
>>Listen, I ran a 292/108 cam in my 367@ .538 lift with 1.6 arms. I ran it with an A833 and 3.55s, for 65mph = almost 2900. Not a bad combo. but I was no fan of that rpm, nor the terrible fuel-economy.
So I installed an A833 overdrive which dropped the Rs to 2000, and guess what, the thing burned even more gas. That led me to figuring out why, and that is how I got to the above.
So I pulled that way-too-big-cam outta there in favor of a 270/276/110 cam And right away I was in business, so much so that I bought/installed a GVod, and dropped the cruise-rpm to 75= 1850, and that combo made the most mpgs I ever measured (with my Barracuda). But I tuned it to put the pressure peak right where it needed to be, then kept it there until the gas tank ran dry.