If you have done modern cars,the late 30's and 40's stuff will be like a day at the beach to you. Much simplier cars and easier to work on. Just different.
Well, removing the rear brake drums can be a problem,but it's nothing the proper puller and a huge amount of patience can't defeat. With the proper amount of swearing,throwing stuff around the shop,and a short vacation once they are removed added,of course.
You were told to look for a complete 46 or later engine because they are more modern designs and none of the major internal parts will work on your 201 engine. It's much simpler and cheaper to just rebuilt a 46 to 59 flat 6 Plymouth or Dodge engine and bolt it in than it would be to pay somebody to modify and install install the modern crank,rods,and pistons in your engine,or to modify your block to accept them. IF that were even possible,which it isn't.
I would suggest you keep your original engine stored away somewhere in case you ever decide to restore your car. Check the engine number first to see if it matches the number on the title. If it doesn't,it's not the original engine anyhow,so you have nothing to lose by installing a more modern flat 6. Your new Hollanders will tell you everything you need to know about 1939 serial numbers and where to find them.