Anyone run into an issue with an aftermarket heater core for 68 SB no a/c being THICKER than the original Harrison core? I've purchased the core from the local O'Reilly's in my area, but upon inspection it's about 5/8" thicker. I didn't figure this would be a problem however the retainer clip won't screw down. It's at an upward angle because the tanks are taller. I've verified part numbers and it's correct one, tubes look bent correctly and correct size, I just can't get the retainer clip installed. Is this critical? The core is flush with the heater box so after sealing kit is installed I don't think it will move around or anything. Thanks for the assist.