i put a bored out standard t/b on my 5 lt calais .it is 70 mm . didnt notice any great gain and it has a whistle caused by the air rushing past the two hole for the carcoal canister ..instead of them being tiny they are now about 5 mm across .
Yeah either go different manifold setup with single throttle body on top - or maybe a Starr setup (although hard to find) if u still like the factory look. Depends if you're looking for a different look.
Incidentally, I have just bought a Walky setup for my 355 that cost a small fortune!!! So I'm selling my Redline and forward facing plenum and t/b setup. If you want pics just PM or email me mate.