355 referred to the torque produced by the motor. It was 300 cubic inches. "Mill" is just another name for motor. You should be able to find parts for it most anywhere.

What Pope was suggesting is to simply find another Buick motor that is in good running condition and bolt it in. That would be the cheapest way to get the car running good again. Check with your local recycling (junk) yards. Look in the yellow pages under auto parts, used.