I had a 33 Plymouth Convertible from 1973 to 2012 with stock flathead 6 and 38 Desoto overdrive. I drove it 110,000 miles frequently at 60 mph (100 kmh) on the flat roads.
I also have a 1935 Dodge Pickup (early suicide door model) with a 1959 Plymouth 230 flathead 6 and a T5 Overdrive transmission). It will do 70 mph on the flat roads. I have driven it 95,000 miles since 1990. The 35 Dodge pickup has a front end identical to your 33 Dodge including the engine compartment size.
I looked into the long block flathead 6 and it wouldn't fit without cutting the firewall and moving the frame cross member consequently I would recommend that you bore a 230 before going to the long block. The stock compression ratio is a little over 8 and so after you rebuild it expect something slightly higher.......suggest staying in that range to use the gas with octanes around 86 or so. The five speed T5 transmission is an easy install and will give you not only overdrive but also the extra gears to keep the engine in the rpm range for best torque when pulling on hills.