OK, I uncovered the truck and took some pics for you, the truck is a 56 panel with a 350/350, virtually identical to your 58 [and mine] except for a slightly different shape core support
yes, the
shroud extends 6", to put the fan exactly in the opening, it's the shroud all the vendors sell for the 55-59 V8 conversion and can be found on lots of'em .... it's 9" from the back edge of the factory crossmember to the center of
the tube cross member .... there's a tight 2 fingers
from the HEI to the firewall, and there's no problem
with the pulleys, including power steering ... and there is a 1" spacer between the water pump and fan
I'd expect that if your fan isn't pretty much parallel to the face of the rad core, your front mounts are too high, putting the engine slightly more forward to give enough dist clearance - at any rate, the fan
must be within 1/2" of the rad, however you accomplish that or you will have overheating problems
Bill
