What the f*ck are you doing selling that Guzzi? It's beautiful!!! Only decent bike the wogs ever made, well their stuff might be better now, but I had an old bevel drive 900SS, and it was the biggest heap of #$%* that God ever shovelled guts into! It did look nice though, I spent a lot of time looking at it, and swearing...............
My best mate is a Guzzi nut, he's had a T3 and a Mk5 Lemon, and they were great bikes. He had a break for awhile and put his toe in the water and bought a CB750K2, and recently a Suzuki GS1000 (with my blessing, of course!) but is craving another old Guzzi, maybe an SP1000 or even a "Pasta Glide", this time? I'll be happy too, I'll "inherit" at least one his other bikes, and get to ride the Guzzi on occasions too.
I'll send you a pic of my NOS alloy tank I've got for my K2 racer project, it's a 32 litre "endurance racer" tank from the 1970's, (geez, that's about 42 bucks worth of gas!) it looks great, but is awfully big (much bigger than the RC750 tanks) and the last thing you want is something that's gonna dwarf your engine, which is physically pretty small anyway.
The 350/4 tank is a good option, they are an attractive looking tank, or even buying another 400/4 tank and "cutting and shutting" "knee-dents" and "clip on cutaways" into it'd be good, with maybe a modern race type filler? Keep that Guzzi though mate, now that's a mans bike! Cheers, Terry.