dually towing

[bigblockdually]

hey guys i was wondering if anyone knows how much an 87 k30 reg cab 7.4L turbo400 4.10 gear dually with cab lights pwr door locks pwr windows am/fm with cassete radio! custom made towing hitch which is all sorts of tied into the frame and bumper (fabricated by a machine shop mostly 1/2 inch steel) can tow? oh yea its got a/c heat dual batterys 12,000bs wich on the front which is also all sorts of tied into the frame (also made by machine shop) silverado because all this additional random info has no bearing on the subject of towing except for the bumper and driveline specs oh yea and has a trans cooler with fan and tilt steering and cruise control. so yea i want to bumper tow a 13,000 pound trailer 365 miles and was just wondering if it is going to pull the wheels of the ground because i know the hitch isnt going anywhere