If it doesn't cost too much, that is not too many joists, I'd almost take out the existing X bracing and sister a same sized joist to each existing one. I suspect getting them on top of each wall at the ends would be impossible. Would need to do that on one end and fasten something solid to the wall under the other end of the joist. Use polyurethane adhesive and 3 inch screws between the old and new joist and then put in new X bracing. That would avoid a block under them and the attendant head banging.

Would even be good if you found joists with a small crown and put the convex side up, (of course).

Upon re-reading of the OP, I may have got it wrong. Somehow I thought you were talking about the largest room having some bounce. Sistering joist for the whole house would be a tall order.

One thing that might be superior to X bracing is using blocks of the same stock of the joists, say 2x10 and just fill the space completey. Probably a tad more firm than the X bracing and should last longer.

__________________
1986 300SDL, 362K
1984 300D, 138K