Erick! you're back! 12' is over spanned for a floor load bearing 2X8! Break up the span and sister on another 2X8. Install your sister joists. Clip the nails off with some end nips, wire cutters that are flat on the front, not the side. No sawzall...too much opportunity for metal n the eyes. Crown the new material (look for a deflection and set the high point up). Pre-rip the edge of the 2X8 at a 45 degree angle and tip it up into place bearing 2" on each end. The angle goes up! You'll need a mini-sledge at least! Construction adhesive may help with noise, but engineers discount it. Fasteners are 16D nails, three every 8"", not 16". Jack up the joist before fastening. A compressor, hose and palm nailer make short work of this. You can cut your 2X8 into two pieces for easier installation if you..... Set a girder line at the 6' center (if possible). A 4X6 with posts and footings every 6' is adequate. Pull the bridging and block line the top of the girder. 12'X12"X12" footings at a minimum. Jack up the girder line before setting 4X4 posts from girder to footing. Make sure you have at least 12" of clearance under the girder! I did a fairly in depth run through of this operation in a post previously...I've moved well past that post, maybe Lizbeth still has it in her conversations? Good Luck!