Matt, I am no expert but I did build my share of Pontiac musclecars through the 80's and have done plenty of mixing and matching myself. With a 16 code on the exhaust ports the '68 heads sound right, but the small plugs really do bother me.
The '77 block is fine, although I always try to keep 60's parts in 60's cars just because. But 70's heads have a much lower compression ratio and definitely don't belong in a classic goat. The old heads sometimes disappear during engine swaps because they are valuable. The LeMans convertible that I have now is a rare "Sport" version and came with the 325 HP 350. The previous owner (clue-less) was very proud that he had put late 70's "Trans Am" heads on it when he had the engine rebuilt. I figure that the head swap was the shop's idea, and I am sure they made some good money on the old heads. Needless to say, the 350 with big-chambered 400 heads was a real dog and tossing them was one of the first things I did.
Which goes back to your situation. If you have the original heads on the car then the C/R will be very high for today's gas and it should want to ping and run-on especially if you try putting regular in it. This also puts quite a load on the starter motor when the engine is hot which is hard not to notice. The 70's compression is much lower which will make the engine much happier on pump gas, but this also kills power especially at higher RPMs. If you have the original heads and a reasonable cam it should pull strong through 5,000 RPM. If power quickly dies back over 4,000 RPM that's not a good sign. So how the engine actually runs should provide some clues.
By the way if your 400 had the '68 350 heads then the C/R would end up at almost 12:1 which would create big problems, so this is highly unlikely.
Other things to look for- although the 70's heads will bolt up the heat crossover passages don't always match up correctly with the 60's intake manifolds. If you have the original 60's intake and an exhaust leak at the crossover that's also not a good sign. Let's see, another thing worth doing is pulling a valve cover to see if the heads have screw-in studs. To be honest I don't remember if the 16's came with them and there were plenty of excellent '60s heads that didn't, but its another thing to add to the ID list.