Over the last few months I've been watching and measuring the ride height on my 72 LT1. Much to my concern, I'm having a lot of trouble getting the ride height to the correct measurement. In fact I have it as close as I can and I'm still about 7/8" off, left from right. I have one bolt to the top, and the other all the way out and still can't get the spring to even out.

So, here's my question, back in March of '94, the car had an F41 spring installed, there's only been about 25K miles put on the car since then. Is it time to replace it?   I would think these things last a little longer !!!       Any thoughts?

I doubt that the spring is 'worn out' or fatuiged in such a short time. It's possible the spring was 'bad' to begin with. It does happen.
If you look at the spring from the rear, it should be almost level all the way across, from one side to the other. If it is, and you have a ride height difference of 7/8" from one side to the other, I would be wonderin' about possible frame damage, or bad bodywork somewhere down the line. Maybe time for new body mounts?
Is there a difference in the front, also? Adams' Apple38557.5914351852

Mine is off around 1/2 inch. When I replaced my spring a year or so ago I was told that you should not use the spring adjustment to try to even out right to left ride height. Use the adjustment to get your rear ride height where you want it but keep both sides relatively close to the same place on their bolts. If it is really noticable, sugest looking at the body mounts as a possibility.

Measure from the frame to the ground.  If it's even there, the body mounts are the problem.  If not, you have a suspension/spring problem.