1966 A100 with a Slant gas gussler. HELP!

I bet those tires are about 28 tall.
In which case 65=2285rpm
And pushing that brick down the road, the 1bbl is gonna be pretty far open,
Meaning perhaps all the carb circuits are flowing, most to the max.A different carb, at this point in the diagnostic, is pointless.
At 2285 rpm, and the throttle-valve quite far open,the timing is gonna need to be fairly conservative, else it may detonate itself into an early grave.
Your 2.93s may have pushed the slanty into an rpm where there just ain't much power.
Back to basics.
Make sure the cooling system gets up to a minimum 180*F. If the cooling system can't get rid of heat at 180, fix the cooling system. I would recommend an even higher temp. I run 205 on my 367.
But before you spend a ton of money on the cooling system, just get the temp up to 180*F, in preparation for doing a hot compression test.
So do the compression test,at WOT. Then lash the valves about .002 looser than spec. Make sure you measure it right. The rocker arms on slantys are a little soft and the valve stems like to hammer grooves into them, making lash settings tricky.You need some narrow feelers to get into the grooves.
Then warm it back up and repeat the compression test. Why again or why two? Well then you can see if the lash was sticking a valve open. If nothing changes and it's bad, then we have to do a leakdown test. But if the second is better than the first then you are a witness to the change. Why the extra .002 lash? Because, you are working that slanty hard, and you want to give the valves half a chance to cool, which they can only do whey they are touching the valve seat.
OK, the FSM says good compression is 110 to 140 with a maximum variation of 20 psi. I can tell you this; at 110psi on every single cylinder, you will not get any better mileage than your mediocre 10mpg. And so you must hope for the 140#,lol.
The factory compression ratio of those old slantys is listed at 8.4/8.5.
Believe it or not, when I punch the slanty numbers into the Wallace calculator, it comes up at 145psi.That doesn't mean any of 'em ever made that; it just means that's what the numbers come to, giving us hope,lol.
And I would be looking for a tighter than 20 psi range. I like when they come in at less than half that.
So, go see if you even have an engine left.
After the test results are in, then we can formulate some new tests.......or schedule some engine work,lol.