putting up new drywall, Old ceiling joists bowed-

I have been working on the foyer of my 200+year old home. I am almost done and ready to start dry walling. The 2nd floor floor joists (the ceiling joists in the foyer) are significantly bowed. I just replaced the 1st floor floor joists (sistered new ones in) but don't plan on doing the 2nd floor ones. but I am concerned attaching drywall to these would not look good. The old ceiling was a drop ceiling so this wasn't an issue. The span is 15' 2" and the joists are rough cut true 2x8. I started to put up some shims on one end since it wasn't too bad. the one picture shows a 1/2" shim on end, then 3/8", then 1/4", which continues for a long distance to about mid span. But the other end was really bad at 1 1/2" and really too much to do this. So I figured I would need to sister in a new nailer. other pictures show it in place and how far out the joist is on the end. Putting these are a lot easier than shimming. Is that the way to go? I was originally concerned about all the extra weight since the joists are bowed which is why I started shimming. but it is too much out to do it that way. I could scribe the 2x4, then cut it and screw it up on bottom of joist to avoid the extra weight. will need longer drywall screws to go through this 1 1/2" "shim" to do this. I don't think the joists are moving anymore? think nailers whole length OK? That is easist.

 

I would sister the joists. It would be great if you can go full length so your sisters will also provide some structural support.

 

I could go full length, although the task of finding 10 flat 2x4x16' may be daunting! Finding flat 8' ones wasn't so bad. But when i sistered the floor using 2x8x16' it took some digging to find enough good ones. I think the 2x4 will be harder as will be twisted?

I guess I could buy 2x8's and cut in half with table saw if hard finding flat 2x4's

 

Since the joists are sagging I'd go for something more structurally sized than 2x4. 2x8 would be nice for the extra strength.