Fit the Chevy and bollocks to authenticity.
I have never really understood why you would want to make a car that is clearly not an original Cobra, pretend to be one. No matter how hard you try you will never hide the fact that is made of fibreglass and has Jaguar or Ford suspension underneath.
I built a Dax because I wanted a Dax, Not a Cobra. I have fitted Dax badges on it too and I haven't put the word "Cobra" anywhere on it. If people ask me what it is I tell them it's a Dax.
Fitting a Chevy in a true replica like a Kirkham would be wrong, Even I would agree with that but then I would also say fitting a 351W or a 351C in a 427 Kirkham would also be wrong. If you have a Kirkham it has to be a big block Ford looking like the original 427 side oiler.
Just my opinion off course but fit what you like, It's your car.
Built a Dax once, Long time ago, painted it blue, original huh...?