Recently purchased top of the line shower doors to install in original 65 year old bathtub that is in perfect shape. Problem is that the bathtub is not level. There is a 3/4" gap on the drain end of the tub. Any ideas on how to fill the gap?

 

I am assuming the gap is on top of the tub rim at the wall, is that correct. It could be at the bottom of the tub, but doubtful, since I am sure it is cast iron and weighs a ton. When installing the shower doors, although there may be a gap, you should cut the bottom rail the length of the tub minus 1 1/16th inch, and the top rail should be cut to fit exact. Now if the walls are not plumb, you could feasibly have a gap at one end or the other. The side rails have to fit up against the walls, even though one is probably cantering inward the 3/4". Now, you can cut a beveled piece of wood wide enough to place behind the offending side rail, but it should be sealed and painted before installing the doors.
Worst case scenario without installing the wood shim is the doors will not hit the wall evenly. You can adjust half of that out with the top rollers. Move one upward and the other one downward to canter the door flush with the wall. It will be slightly noticeable where the two doors meet in the middle, but there's not much you can do without building out the wall.
Repost with more information if necessary, and we'll take another stab at it.