Not telling you what you should do, or "Should have done" but before interior, exterior or even getting the new Bullet built, I would have taken a Block with the heads on it and made sure the hole was big enough. And what else fits in the hole with the block . . .
jus' sayin'
My Street Rod is kind of always in a state of Flux, so to speak (Re:Header story) and because those old shitboxes were really, really small inside, I've learned that we should always "Dummy it up" before buying new expensive pieces . . . that may not fit . .