I'm not exactly sure what a 9503 carb is. I think it's an AFB type carb...at least according to what Carter's rebuild kits fit.

Anyway, it'll fit if you have a squarebore manifold like an Edelbrock 2101. Usually the stock carb on a 307 would be a Quadrajet and it's a spreadbore (if it has a 4-barrel carb on it already.) If it has a q-jet, you'll need an adapter or a different manifold and hookin up the fuel line and carb linkage will require a little engineering. But it ain't hard to do.

I run a Carter 625 cfm AFB carb (I don't know the model number...it's an electric choke) on my 292 I6 and love it.

hth,

Ray
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