I changed a 70 Nova and a 69 Camaro from I6 to v8 many years ago. I could never imaging going the other way except for an OEM restoration.(if even that).
I say the others are right here, you have a problem with the engine or possibly the transmission. Also, a 350 will give better power than the 283 or a 292, therefore not have to work as hard=better mileage(under civilized driving conditions
)