Renovating an old saltbox style house (1850's). Found the second story ceiling was bowed. The plaster had been removed and covered with sheetrock at some point. A crawl in the attic revieled 4X4 joists attached to the rafters, which are 3 feet on center, about 6.5 feet up the slope, with a span of 15 feet (no wonder they bounced) They were then filled with blown in insulation. Removed all the insulation and pulled down the sheetrock and lath. Two questions: 1) I would like to move the ceiling joists up about six inches for more headroom and replace the 4x4's with 2x8s. Is this feasable as they are not attached to anything structural ? 2) Can I run furring strip perpindicular every 12 inches on center across the joists (remember they are 3 foot on center) and hang new 5/8ths sheetrock without it bowing?