There's a guy selling a 4" exhaust with flowmaster 50 deltas for a steal.
Is this too much for a mild big block?

I would say yea It's too much. If your not going down the track and need that extra .5 second and have a blower, I still think no.

too much.
+ wicked ground clearance issues.

4 inch with a delta flow 50's? bit of a wierd combination, but still WAAAYYY to much for a street BB, personally I think my 3's are overkill.

The real power loss/gain in your exhaust system is in the first six feet of tubing downstream of the exhaust port. If you have done your homework well in this area adding a huge tailpipe/muffler setup will have no effect other than more weight, more noise and less ground clearance.