Fenderwells

well, i got the 440 in my car to mock it up and tried to put the fenderwell headers i got on it. so i started with the passenger side. i started with a small hole in the fenderwell te gradually made it bigger to fit the header through. using pics that i got off of this site i was able to get an idea of where i need to cut. i got the header slid into place on the passenger side with the engine in the car. the problem is, is that the header is physically too short to clear the body. the tubess coming off pf the flange look like they will work, may need to move one tube to clear the upper shock stud. but the tubes that go inside the fenderwell and extend under the car are not quit long and are causing the header to hit on the collector and make the entire header sit at an angle, preventing it from bolting to the engine. its about 3inches too short. ive made headers in the past for srteet rods and such and have no problem modifying these to fit. my question is, would these headers be for an earlier a-body? they look almost identical to other fenderwells i see on a-bodies of my same year. or could it be that i need to cut the support gusset plate that is directly under the firewall? thanks.