Won't start

If your slanty idles at 6",something is wrong. Maybe the gauge is boofed...or maybe it's plumbed wrong.
If the gauge is accurate,in all likelyhood the timing is off, no matter what the marks say.
But of course, you want to verify the marks first......then hunt for a vacuum leak ....And then make sure the valve lash is correct.....And then we check the cam-timing.......
So, common sense first;
Does it start right up, both cold and hot? If yes this would tend to rule out the valve lash. and the cam-timing.
Does it idle nice at 650 rpm, and how about when you put it into gear? If yes, this would tend to rule out cam timing and compression issues as well;and additionally a vacuum leak.
And how well does it take a load, meaning can you drive away briskly without bucking farting or stalling? This would tend to rule out all issues, except bad timing marks.

Ok so, the easy test then would be to start her up and advance the timing some,with the vacuum gauge installed ,plumbed to MANIFOLD VACUUM, not the sparkport. If the rpm goes up, then that's a good thing, continue advancing the timing;forget what the marks say,continue this way until either;A) it stops idling faster, or B)the vacuum rises closer to 18/20, or C)you run out of ability to advance it any further. Now take it for a gentle spin, listening for detonation, while exercising the gas pedal. How is the power level? Better? Then great.How about Detonation? None? Great. How about at 2000rpm and floored? Still no detonation? Supergreat! Go back to the garage.
OK, with no detonation and increased power with the additional timing, we are on the right track. How's it start now? Click,vroom? Or Rrrrrrrurrrurruirrrrr; that's bad. Or starter kick-back;also bad. If you get either of those starting issues or experienced detonation under acceleration, then you are gonna have to put the timing back, and prove the balancer mark is accurate.
And if it is; then you are on the hunt for a vacuum leak.

But,heehee,
if you measured 6* at the sparkport where the "pod" hooks up, well, then, you can ignore everything above in this post,lol.
That port should be dead, zero, no vacuum at all. The fact that you are measuring 6* speaks to the throttle being waaaay too far open, and that speaks to the timing being waaay retarded, or the engine being waaaay lazy. What do you have the idle speed set at?