I thought 1946 GMC tailights were the same as Chevy. If so, they are not round (as you state). Perhaps you have the correct brackets but the wrong lights?
Kent


1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix