I think your answer is in this 1948-1951 Chevrolet Truck Shop Service Manual.
Under Chassis Sheet Metal, chap 11, pages 3-4. Pay special attention to page 4, Lower Catch Plate:-adjustment-par 2.
You may have to take a small screwdriver or wedge thru the grill opening and push the
lock bolt one way or another while holding the latch in the open position.
Once you get it open you should make the adjustment outlined in the manual.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1103.htm http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1104.htm If you can’t get the lower catch plate to center on the lock bolt then the upper catch plate
may be bent. I use a short length of threaded rod and a lock nut in the weld nut of the
upper plate and tap it back in alignment with a soft hammer, then you can adjust the
lower plate according to the manual.
Denny Graham
Sandwich, IL