I think Adrian has already ported his head to match a 36mm dellorto, so you can check with him. I am not sure if you'll loose any bottom end grunt tho.
Both of my AVL engined bikes, the Electra-X and ASBO 12 run 36mm carbs, The Electra has a Mikuni TM36-31, ASBO 12 has the Dell'Orto which I still need to tune properly for top end performance. Bottom end performance/rideability on both is absolutely fine, I couldn't feel any deterioration when the Electra went from a 32 to the 36mm carb.
On both bikes all I have really done on the inlet side is to open up the port at the carb flange just enough to match the 36mm carbs, and blending the now 36mm round hole into the 32mm x 36mm oval section of the inlet tract along the depth of the flange, no deeper. Apart from cleaning up the rest of the inlet tract (OK, and some polishing on the Electra) that's it, the port shape is still fundamentally unchanged.
There's probably enough metal in the casting if some maniac wanted to take the inlet out to 38mm/1.5", but I very much doubt there would be anything to be gained. If, however, someone out there has pictures of an Electra-X, AVL Classic or 500 Machismo fitted with an Amal GP3 carburettor I'd love to see them! The GP's flange mounting hole centers aren't too far off those on the AVL heads, 63.5mm versus 60mm. I think I'd better stop there, or Bullet Whisperer might build one...
The EXHAUST ports on the 500 AVL heads are an absolute disgrace though and could use a good clean up if you are running with an un-restricted header pipe.
A.