Dana 60 with upgraded slant six

Even if you turbo a slant, that would mainly help to maintain torque at high rpm (thus HP) and not greatly increase max torque (at lower rpm). I would expect a rear-end is suddenly broken more by torque than by max HP, though high HP will wear the gears faster. Regardless, most 318 V-8 Darts came w/ a 7.25" rear-end, so unlikely a slant would damage one. Better than a 8.75" rear might be a 7.25" w/ Sure-grip, though those are rare. Refer to end of the movie "My Cousin Vinny". If you want to go fast, less weight is as important as more power. Big brakes are good for road-racing, but not smart for the drag strip. When you do need a bigger rear, consider the Ford 8.8 (Explorer, ...). One side must be cut, but many say that route is cheaper, plus gives rear disks (search).