Yeah, don't jump to too many horrible conclusions...
It's possible you have very little damage at all. Absolutely STOP running it!
Bearings and rings tolerate a lot of brief thermal abuse and more age than get damaged from an overheat or short duration low oil pressure (or condition) event.
One key thing - did you have good (especially normal for the car) oil pressure the last time you ran it? I really hate to sugggest this, but if you didn't notice, you might even consider very breifly running it one more time (under 30 secs) - and at very low rpm - just to see this.
If you choose to do that, refill oil and water first - and notice not only what the oil pressure is but how quickly it rises to normal from start and listen closely for any noises such as lifters or a deep, horrible cyclic thumping noise which could be a rod knocking. Hopefully you already noticed the oil pressure though and do not need to do this....really, really, really try to avoid even turning the motor over much if possible, let alone running it. Knowing your oil pressure situation though will have a huge impact on what all you need to consider doing - so much that you really need to know it even at the risk of adding some more damage.
You will normally hear angry hydraulic lifters with oil problems long before you are losing main bearing condition, which will quickly lower oil pressure. Again, you will normally see very low or largely fluctuating oil pressure (at a given rpm) with damaged or overly worn main bearings before you hear rods knocking from damaged rod bearings. Even more, you normally will hear rods knocking (and more than just one) before you have cam bearing damage. Cam bearing damage is the worst problem because they are such a pain to change in the vehicle.
Note the compression checks very carefully - write down each single one. If you want, you can do the "wet" test with a couple of squirts of thin oil (WD40 works OK) in the cylinder and re running the test, but you must both use a very uniform amount in each cylinder for the information to be of any use and the information is of arguable use any how to determine ring and valve stem seal condition. (I never got much from the wet test and stopped doing them.)
Now, with aluminum heads, you'd be looking closer to some warped heads or at least reseated valves, but less so with iron heads.
Since you need to pull the heads to swap gaskets, take the heads in to a decent machine shop to have them checked for straightness, valves seated right (leaks), valve guides, seals, etc. (If it's been a while, this is a great time for a little 3-angle valve job and new springs - a whole head rebuild - for $300 or so...)
With any luck you didn't damage pistons, scar the bores or even wear out rings, and bearings are the more likely heavily worn - but even that is far from likely. All (except for cam) bearings are easily changed from the bottom. You can pop off a couple of main caps first to see if they look reasonable.
If you have some cross hatching left in all the cylinder bores, you're only left with the decision on whether to go ahead and re-ring at this convenient time.
You need the critical information on compression and oil pressure before getting to this level decision, but should the worst all be true and you have major damage - consider all your options.
Is your car an NCRS numbers matching effort? Is the engine even original? These factor into the consideration as to whether to undertake an expensive extensive rebuilding versus putting in new much more powerful crate engine.
Your '76 came with the weakest engine GM ever made - except for the one mine came with :ugh - and crate engines with stronger components and good heads are available as cheap as a rebuild. A stock '76 ranges what 185-205 hp and similar torque?
Just here in the JEG's catalog laying on the floor GM Performance Parts crate engines range from a modest "target" motor making 290hp/325ftlb for $1600 to 330hp/380ftlb for just $2400. Numerous manufacturers make crate motors up to considerably more power than in the new vette plants for under $6K. There is even the option of easily changing to a big block powerplant and adding a few hundred to those numbers (big blocks fit in '76's easily...)
Should it come down to a major rebuild requiring the engine to be pulled rather than just a head gasket change, consider those options carefully.
