Merv, this is a decision you are going to have to make yourself. Considering the style of Cobra you want to build, I would go with Ford 302w. Not too many Cobra builders have the choice of 302w, so consider yourself lucky that you have a choice.
L76:
The L76 just produces a lot more HP in standard form, it's easier to tweak to get more HP by just simple ECU programming. And if you start changing cams etc, you get even more HP. The Chev engines just have so much HP potential.
The L76 is also a lighter due to the alloy block & heads and much more compact unit due to how snugly the accessories & pullies sit against the block.
If you want to save some money, just look for a 2nd hand engine & box. Or save even more money, go for an earlier engine. You can legally fit the earliest of LS1 engines if you like.
Ford 302w:
Definitely the traditional way to go in a Cobra in Australia. To be honest they produce enough HP with mild changes to make for an enjoyable Cobra. Come standard with Borg Warner T5 gearbox which is a nice gearbox to live with.
You have a choice of imported Mustang HO engines or local AU Falcon engines.
Just recently I've seen mid 90's Mustang HO T5 front cut local to us with 100,000k's for $2500. You can find them cheaper or more expensive, the local 302w engines with gearbox are about the same price in my experience.
My brother went for a local engine, 2002 AUIII Tickford. Nice engine. 220kw standard, hand built engine, this one was by Steve Berry, has
oil cooler, roller rockers etc all standard. The AUIII have the extra hassle of Smartshield security which needs to be incorporated/bypassed/removed.
I expect you have seen Mike Verners aftermarket injection on his 302w. Classic carby looking injection, it's well priced and got that clean retro look, that's the way I would go down the track after rego.