Don't know if you want to avoid the highways but personally I would take the 309 out of Ayuthaya to Wang Noi where you cross HWy 1and then continue on the 352. the 352 is a very long straight road so if yo like twisties this is not the road to be on.
Rd 352 crosses the 305 near Thanyaburi to Lam Luka and puts you on the 3004 to Min Buri. Here head east on the 304 (Suwinthawong Rd) to Chasoengsao - you can also go more or less straight but that puts you on the Motorway smack in front of the airport. Sorry don't ride a bike myself so don't know if they allow bikes on the motorway. In Chasoengsao you can head south on the 314 to the Motorway or either take the 315 to Phanat Nikhom and then the 349 to Nong Chack and then the 2056 to Khlong Kio and the 3138 to Kao Khan Song till you reach the 331 and 3376 to ban Chang.
Alternatively (well there are many alternatives ) or about 6 km after you cross the 304 head east along the 3304 till you reach the 331 to Ban Chang.
I know the main roads pretty well, have a house outside Ayutthaya and have a business down in Ban Chang so drive it quite often but was wondering if anyone knew of a decent ride down the back roads.
Well, could "cook up" something along back roads but explaining the turns to take, often on un-numbered roads would be very difficult so I guess you will have to take your chances - anyway isn't that part of the fun of exploring?
Came back from Mae Sot via Tak to Ban Chang. My destination from the North West was Saraburi for the night so I could shoot down R1 and take a left on R33 to Nakhon Nayok. From Nakhon Nayok you can follow the signs for Prachin Buri and take R319 to Phanom Sarakam and then R304 and turn onto R331 South to Sattahip/Ban Chang/Rayong.
Alternatively, from Nakhon Nayok there is a turning right onto R3347 and onto R3076 to Phanom Sarakam and onto R331.
Hope this helps.