Marty,
Assuming you are referring to traditional size GG1's, ( 13" shell ) there are variations that will and won't fit. Most Lionel shells should interchange but will likely leave you with an empty E unit slot on top. Two Lionel types that probably won't work are early post war and an early '90's variety that MTH made for Lionel. The latter has a wider body,exactly like current Railking offerings.
If the chassis you have is from a scale William's GG1 then I suspect a certain amount of redrilling and shimming will be required to accommodate another manufacturer's shell. I suffer the opposite dilemma here....A scale William's shell without a chassis.
Bruce