opinion on R3/W8 340 (358ci)

"Is it ideal?"
We don't know anything about the car to answer that. The car won't know it's a "nascar" oriented motor which IMO is a little down on power, I'd think around 730-50 would be correct for what those motors were. Will it be a blast? Hell yes it will!
But be prepared for a lot of plumbing with the oil system and headers will be a compromised piece if you still have a factory K member and steering. Nothing a little time and money can't cure.