Back in the day the terms 'small block' and 'big block' carried more weight that they do today. Easy to get big cubes out a small block these days. One part of the question was 'heat'. A 351W bored/stroked to 427 puts out a LOT of it and will rival a traditional big block in that regard. My friends CSX8000 street car with a 'small block' Roush out to 427 cubes is as hot or more so, in terms of cockpit temperature, as my ERA with a traditional big block 427. Without adequate heat shielding (header wrap, insulated firewall, ventilation, etc.) you can melt your tennis shoes, literally, in either car.
Sound is a can of worms and not easily defined. It can vary a bunch depending on side pipes or under car exhaust, type of cam, carb, compression ratio (thats a BIG factor) and other variables. I've heard stock 302's that were as 'loud' as my 427, not necessarily a good thing being loud for louds sake. I prefer a more melodius exhaust note over raw noise myself. One of the BEST exhaust notes I've EVER heard was my own 427 side oiler when it was running 12.5 to 1 compression, comparable to 'back in the day'. That particular 'note' or 'sound' was due in large part to the extreme compression, which was more common in the 60's than we find today. When I reduced the compression to 9.5 to 1 for more 'streetability' it killed the exhaust note substantially. Still sounds like an FE, which is good, but it's just not the same 'earth shaking' raw powerful note that made women hide the children and grown men run to or from the car depending on the 'wuss' factors involved.
For me there are only a few options for engine selection. Small block belongs in an FIA or street type slab side. 427 FE belongs in a SC style 427 Cobra. 428 is acceptable and may even be preferred for a 427 Cobra STREET STYLE interpretation of the orginal.
But since your talking BDR, it doesn't really matter what you run it. What matters is how good a 'story' teller you are. After all you ARE gonna need to explain it. "The superior BMW suspension offers better handling, brakes and lighter weight (yeah, THATS the ticket, your doing fine). The motor really IS a 427, uh, er, well actually it's a 351. The body is pretty close, just slightly wider, longer, but thats what makes it more comfortable, you see, more room. And... (insert other good reasons/excuses/story here)"
Stir that pot, a little more butter on my popcorn please.