If he thought he was breaking things now, what's it gonna be like with that in it...
Worth continuing from NOS's mention of the bellhousing issue: the original Continental engine and Fuller gearbox are SAE 3 (smallest size), most of the engines on my list are SAE 1 (largest size) or SAE 2 (like my C), therefore adapter plates and spacers are necessary which adds to the complexity of the conversion. As Tony also notes, most of those older engines need the bonnet to be extended as they generate their power by being BIG. The newer engines such as Cummins B & C have similar or greater power outputs but in a much more compact unit and revving higher because a lot of their power comes from turbos rather than cubic inches.
In terms of stretched bonnets, this is one of the more hideous ones in my photo collection (dunno what engine it's got though, also note art deco cab roof!):