Do you have a 4 bolt side cover or a 7 bolt side cover?
A 67 Camaro transmission would likely be a full syncro Saganaw transmission (7 bolt side cover). It would not have the same side cover configuration as the original 4 bolt Muncie. The spacing of the shift rods would be different and they likely have to be bent to fit the current side cover. It sounds very likely that the lever(s) are not letting the gears fully engage.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine