A straight six is a longer engine than a V-8. So the problem would more likely be fitting things between the fenders rather than firewall clearance. Depending, of course, on where you mount the engine. But a V-8, with room for a fan up front, should take up less room back to the firewall than a straight six.
Here's a
blown 350 in a AD truck. Lousy picture, but you should be able to get the idea about dimensions.
