You didn't say what compression ratio you have, but in general run the lowest octane the engine will tolerate without pinging. Running high octane for the sake of high octane will do nothing but burn a hole in your wallet.
If you have a stock 1971-1975 engine you can run 87 octane. With a properly setup early (pre 1971) engine, 89 octane should be fine. I've run as high as 9.8:1 compression with a stock cam and 89 octane fuel, and as high as 11.2:1 compression with a hotter cam (increased overlap reduces cylinder pressures) and 93 octane.
Okay, if my previous comments didn't clarify it for you, then here's the low-down. If it doesn't ping with 87 octane, then you can use it. If it pings with 87, but not with 89, then use that. Etc, etc. No reason to use a higher octane except for higher temps/harder usage (such as towing). This will provide a 'cushion'.
Another factor to consider is the engine's condition and tuning. If your engine burns oil, it will need higher octane, as oil in the cylinders will effectively reduce fuel octane, and it may ping. Also, the ignition timing setting and the air/fuel ratio will affect the need for octane. So only you can be the judge of what it needs ultimately.
Fill it the rest of the way up with a container of de-icer (just in case) and premium gas. Once the old gas is out of the tank and you have it decently in tune, fill it with 87 and see if there is a noticable difference. If not, keep running the cheap stuff and save money for other projects. If there is a difference, try 89 and so on. To really tell you may need to get the car out on the highway and drive it.
I recommend heading east, so you call me to rescue you in case of emergency.