SBC with alloy heads is not going to incur the weight penalty you all seem to think - all this c**p about snow ploughs and needing the stopping distance of a 747 is unjustified. Of course you should plan to include improved brakes and suspension into your performance package - for God's sake, with the SBC you are potentially introducing three times the original specs horsepower!
BBC is not really viable for road use - the extra dimensions would make it cramped in the bay and liable to overheating and there is, of course, some extra weight. However, you could start with a "cooking" 396 ci and work your way up to a stroker 500 ci (roughly 6.5 litres up to 8.2 litres). RatZ had the latter and, far from being a snow-plough, was more inclined to stare at the sky for a short period after launch...
An insurance safety certificate would be a wise precauation to ensure that all work has been done to a good standard.