Erick, good question. You're experiencing the joys of remodeling an older structure. Several questions for you before I answer your original question.

1. Even though you held the tub off a 1/2" from the back wall to make the outflow lines work better with the floor framing.......did you provide a ledge for the back tube edge to rest on? If not, you should.

2. Have you checked to see if the back stud wall is indeed out of plumb? If it is, then yes, sistering studs is the quickest way to correct the problem and eliminating the 1/2" offset.

3. Did you set your tub in a mortar support bed and if you did, did you think to provide a barrier between the mortar and your sub floor?

Here's how I set a tub (cast iron or fiberglass). After I have the opening dimensional constraints set, walls plumb, squared and dimensions for tub sized, outflow drains ready to receive the drain assembly, I lay down an isolation barrier. I prefer the Schluter - Ditra barrier, but that's just me. 1. it provides a natural break. Believe me, if anytime in the future you have to or want to remove this tub, you'll appreciate the natural break line between your mortar and the sub floor. 2.) It provides moisture control between the sub floor and your mortar (wet and wood aren't the best environment unless you're wanting to start a mold or some other growth farm hidden away). 3. It provides a thermal break .

Next I will provide a good bed of mortar to support the tub, slightly wetter than I would use for laying brick. You want to be able to settle the tub down into it so that the built in support legs are down on the barrier sheet but the voids are filled with mortar for support.........a couple of good hefty friends are invaluable to help you with this. Then I'll hook the drain up and fill it with water........then I'll nail the flanges and leave till the next morning.

Some may consider it over kill for sure, but, I've never had a call back because the "tub fills spongy when I use it" or sealant failure due to settling. I like to work on the principal that if you can't afford the time or cost to do it right the first time..... you certainly can't afford to do it twice....

Good luck, take your time and think it through.