For the average Avanti a 200R4 is a better choice for an automatic than a 700R4. It has a better spread of gears and can be built to be stronger than necessary for the application. The 700R4 was designed for larger heavier vehicles and performance applications. My ‘70 had a 700R4 and the 1-2 shift was nearly immediate and very abrupt. It was far too easy to spin the tires in any gear.
Another potential issue with the 700R4 is due to its bigger case…it often requires new neutral safety switch and backup light switch arrangement to be fabricated. There are differences in Avanti body dimensions so that need is not universal.
A good transmission builder can take into account the engine size, horsepower and torque, cam specs, rear axle ratio, weight of the vehicle and build a transmission to make for a great driving Avanti.