273 or 225?

As much as I love slant sixes, you're already talking about a V8, so it would be dumb to try to talk you out of that. People build slant sixes because of the love of them and to be different. They'll never be a V8 and lots of people end up disappionted by that, because they jump into the slant 6 not realizing the cylinder head is a huge cork that stops it from making power similar to a V8. But those that know that don't care and build slant sixes for the love of them. I can honestly say I will never put a V8 in my car now. I am just drawn to the slant 6 somehow.

Having said all that, my advice to the OP would be to find a later 5.2 Magnum and build a mild Magnum. You'll find a core in better condition and have better heads, more compression and 45 more cubic inches. So unless you already have a 273, that's the direction that I would go.