Fuel filters can be installed either way, however you want to install it so the fuel passes over the outside of the filtering element instead of the inside, gives a greater surface area for filtration. In metal ones you have to rely on the arrow or inlet written on the can. Like wise never heard about installing on an angle.
Putting a filter before the pump creates a restriction, it only has atmospheric pressure to push fuel through, better to have on the pressure side of pump. We had a problem with a local GM V8 running out of fuel when given a hard time, swapping the filter from before the pump to after fixed this problem.
In the later Aussie Chrysler Valiants the fuel filters had an extra nipple for returning the fuel back to a point before the fuel pump, this had the advantage of allowing the pump to keep pumping cooler fuel through the pump and not allow the heat of the motor to vaporise the fuel in the pump.