I gave this quite a bit of thought before I built my '34 Ford C Gas Roadster. I opted for Du Pont FE-36 in cockpit, and Firefox foam in engine bay. Reasoning, in case you have an accidental discharge in cockpit, you don't have a mess. But most importantly, with foam in the engine bay, if you put a rod out thru the block, you are going to have a fair amount of oil going on to hot headers. If using Halon in engine bay, it will put out the fire but once discharged, fire may reignite. The foam not only puts out fire but quenches headers too.
John