Well I just looked at my other two cabs and they do have a larger area under the dash to accomodate the larger 58-59 wiper motor. I can see by looking in the air vent in front of windshield that the cabs are different. The cab I am using is a big window cab from a 1.5 ton GMC. I changed the dash to chevy and filled all the extra holes in the firewall but never thought about the wiper motor.
I do have a motor from a 57 that will fit. (I think) so ill clean it up and put it in. Its a shame the 59 wiper motor is really nice and complete.
Thanks for your help.
Brian M