Slant six to small block 340-360 engine won't fit.

Yeah wrong oil pan. That's a truck pan. You need a car pan.
It's a truck or van pan. The mounts are the same for a truck or car, but the mounting brackets are different. Also, the 340-360 brackets are different from the other LA motors. I was able to bend my 318 brackets to fit my 340 installation in my van. In a A-Body, I think the truck brackets will set the motor to high.