I have a 98 Celsior with air suspension that recently had the front left shock go bad. I decided to order a front conversion kit to springs however the back would still be on air ride temporarily. Is there a way to cap the front air lines so they dont just leak air? Would the rear air ride still work? Any help is appreciated!

good question, one that i'm not sure was ever considered over the years!
i don't see why you can't just can cap the front lines, assuming you can find the right fitting there to do so. if you cannot find a fitting, maybe you can find the valve block where the lines go to and cap there.
the height is measured by a level sensor so technically, the compressor and thus the rear bags should continue to work like normal - only one way to find out for certain!