Often times the early 911s that are converted, started out as 911T models that had 110 hp, stock. Usually teh cost of rebuilding the stock engine, or restoring the whole car, is considerably more than the car ends up being worth. So they tend to be hot rod fodder.
The Chevy engine does weigh more than the stock 911 engine, but less than the 911 Turbo engine, with intercooler and all. The Chevy V8 with cast iron heads weighs 490 lbs. The 930 engine weighs 510 lbs. The standard Porsche 911 engine weighs 410 lbs. An aluminum Chevy LS1 weighs 400 lbs.
A good site on the process:http://www.renegadehybrids.com/911.htm
Costs of a complete 911 turbo engine are really, really high. Even a 200 hp 3.2 liter NA engine will run you well over $3500 used.
The weight bias will be no worse than a 911 Turbo. In fact, it could be a bit better, considering the slight added mass of the radiator up front.
So, it's cheaper, faster, and handles at least as good. Not a bad combo.