to start, since you say the gaps are not even close to same, check out the cab mounts and also the rad support mounts. the cab sits on rubber at the rear and steel on steel at the front, then the rad support sits on rubber. if these rubber mounts are hard, cracked or missing then the cab can be allowed to sag down at the rear and this can affect the gaps. same for the rad support. also check the steel around the cab mounts to make sure the area is not rusted out. these trucks tended to get rain or snow melt caught in the inner cowl vent plenum that runs across the front of the cab by the windshield and then down each side to the floor vents and end up draining just below the vents. the drain hole is small and also has a little flap of steel screwed on to partially block the hole further. over time leaves, pine needles or ice can block the hole so rain or snow melt can build up to the point that it drained out through the floor vent out onto the floor of the cab, pooled there or ran down the step area and pooled there until it could dry or get out the door to step seal. this has rotted out more than a few task force trucks and that is why there are replacement parts available for the lower hinge pillar area, front cab mount area and the step and rocker area.
if those areas are all good, sheet metal wise, i would check the assembly manual fot the frame drawings and get the dimension between the floor of the cab and the frame. then check that against your truck. get the cab correct and then check the rad support mounts and get that area correct. after that the discrepancy for the inner fenders is likely just a manufactture flaw. the assembly manual will also show the correct assembly of the anti squeak foam and the bolt required at the firewall mount lower "L" brackets.
the assembly manual can be downloaded free at the trifve site. google
55-59 chevrolet truck factory assembly manual
and click on the trifive site
or try this link
https://www.google.com/url?sa=t&sour...hgNerbXG4oM_J-