Hi - i recently had a SS top pipe made up from my rusty old pipe, after searching high & low for a new Fiat part to no avail. I would agree with S130 re SS fabrication, its relatively cheap and a good long term option. If you do this just make sure to get the raised lips at the end of the outlets to give the hoses something to hold onto. Looks pretty good too !
HOWEVER would also consider the in-head thermostat option, would alter the standard top-pipe arrangement? but result in a cooler twin cam - I wish i had done this in hindsight !