What vent type and configuration does my furnace use?
You know which way your filter is installed, right? You'd need to in order to change it. That tells you flow direction. So does the air come out of your furnace at the top or bottom?
Most are upflow, and I suspect the same here. The lower panel covers the fan and fan motor, which drives air up through the heat exchanger and out the top. Sound right?
Be aware that modern furnaces tend to be more complex than those from the era you're replacing, and as such can require more knowledge to set up and tune for your duct system. Also, most are going to be direct-vent types that need different exhaust venting than what you have now. You may want to employ an HVAC technician for this project.