I know this is the HIPO shop, but here goes anyway... Try driving the truck with the 6 cylinder the way the factory made it. You just might like it. (I might be a pot here calling the kettle black)
Rear ends are a dime a dozen. There are many to chose from. I have an IMCA modified full floating 9 inch im my 59 gmc 1/2 ton. I swapped a 1959 pontiac 389 motor for the original 336 v8 the gmc had. The 389 was bored and stroked to 447 with HP around 325 and torque around 500 lbs. The factory axle did not like the bigger motor... The perches were ripped clean off the axle, but not at the welds. They tore the axle tubes apart when they came off.
