There is usally a build sheet under the floor mat or carpet. If I remember correctly the F250 & 350 6 with an auto or manual with a 4.11 rear was rated for 6000 lbs. The trans on the 300 302 and 351 s I believe are the same the bell hsgs are. 351, 400Ms & 460 have the same bell hsg s. There are a few cheap things that can be done to the 300 for quit a jump in power and economy.