G78-15 = P215/75R-15 H78-15 = P225/75R-15 L78-15 = P235/75R-15
H70-15 = P235/70R-15
With your current tire size the equivalent of the standard, usually non-a/c car, it's plenty good enough to hold the weight of your car. The a/c cars got the H78-15 tires, so that would be a common upgrade. The L78-15 was the largest tire, other than the L84-15 Chrysler used (that other brands usually didn't), back then, normally on station wagons.
P215/75R-15s are pretty easy to find. The P225/75R-15 is getting harder, but still around in a few brands. The P235/75R-15 is generally available, too.
As for an upgrade for your car? The P225/75R-15s would be my particular choice for non-wagon C-bodies. It's got plenty of weight carrying capacity to have a reserve at max inflation pressure and max-rated vehicle load situations. They tend to fit the styling of the wheel wells and such, too, at factory ride height. An alternative might be P235/70R-15, which would be the same approximate size. I suspect the wheels are 15x6? But could be 15x5.5 .
One thing to consider is that as your car currently has P215/75R-15s, that's what the speedometer gears are calibrated for (gear combination in the trans tailshaft housing). The larger sizes will make the speedometer indicate "slow" for actual speed, similar with the odometer. Nothing that can't be easily fixed by somebody that has the correct gears to make it work (according to what the Factory Service Manual indicates for the original tire size).
ALL sizes will wear well if the inflation pressures are maintained in the 30+psi range and alignment is in factory specs. My personal pref is 32/30, f/r.
If you're looking for improved handling and steering response, stay with the P215/75R-15 tires and get some 7" rims for the car. I have that combination on my '80 Newport (using Magnum GT wheels) and it surprised me. What happens is that the sidewalls are more vertical when installed on that wider rim width and that relates to quicker steering response, as there is less flex in the sidewall. With the wider rim width, that also opens up the possibility of using P225/70R-15 tires (some are still in WW, most in RWL), if desired. But after I discovered the benefits of the wider rim on my '80 Newport and the stock spec P215s, I saw no need to get the wider tires later on. Saving money and enjoyint the ride more!
CBODY67
to get the Hankook Optimo 235-75-15 on most of my cars, and one of the reasons is because they have a 108 load rating instead of the usual 105 rating (the latter being able to carry less load). They are also accepted in Europe. 
