Merv, I don't know much about the 302, suffice to say it was never released on any of Shelby's Cobra variants as far as I know. I guess the badge is the same but that would be about the only thing on a Harrison.
How is a 302 easier to fit? Eg, have you considered the mods required to adapt the L76 engine mount pickups? Also you've got the widened T56 tunnel - so you'd be giving up valuable cockpit space for no reason.
If it's a money decision then I'm sure you can fit a used 302 for less than an L76. Money aside, I'd be thinking ease of build, aftermarket options and performance. I don't buy into the Ford/GM argument with replicas because they're not Cobras... they're just hotrods with Cobra shaped bodies.
Having said all that, the engine is the heart and sole of these cars and logic needn't apply. I say go with your gut feeling.