Tension is determined by scale length, pitch, and linear density (mass per unit length). Most strings of a given type have approximately the same volumetric density, so look for for a set with larger diameters (gauges) to achieve higher linear density. Kalium has a string-tension chart for any scale length you choose, and it should be approximately correct for other roundwound strings. I've used La Bella sets with E strings of 0.110 and 0.115 on my SG to achieve this effect. Getting the right winding length will be important. BassStringsOnline has good page with the winding lengths of various brands' different lengths. Can anyone with a Mustang measure the distance from the ball to the far edge of the nut?