Trouble tuning 340.

the 71 engine with the 360 heads and .039 headgaskets, should have a compression ratio of about 10/1.
There is no street cam that you could install even moderately close to correctly, in your 340 that could generate pressures as low a 100psi, or even 130psi.
My guess is that your gauge is in error, or your rings are capitalD- DRY, or you did not perform the Compression test correctly. So I urge you to repeat the test with a different screw-in gauge.
Reset the lash to what has been suggested; lightly oil the rings as has been suggested, and then pump out the excess, then; crank the engine with all sparkplugs out and the throttle at WOT. Crank as long as it take to see two successive same or nearly the same, pressure readings. Make sure you start with a fully charged and properly working battery, and that the cranking speed at the end of the test is about the same as when you started.
If your pressure with a different gauge is still as low as 100psi, then you are correct, your pressure is way to low,and your low-rpm performance will be very disappointing.
If the pressures are all very close to the same, then we can look at cam-timing, but more likely a cam-replacement is in your future.
If the pressures are NOT all very similar, then I would recommend a Leak-Down test.
If the results of that test are pitiful, then I would recommend to remove the valve gear, and do the LD test with the pistons at the bottom of the bores instead of at the top.