Hi seems like these questions are always asked ,1940 chevy coupe has a 113 inch wheelbase. Some things to consider before you start:
Need to find a donor with that wheelbase or you'll be cutting frame lenghting or shortning
Trac width of the donor compared to original
The arch of the donor frame,you might be cutting half the floor out to get the body to fit on frame,plus body mounts to refit to frame
Front sheetmetal what will it take to fit the front sheetmetal back to the donor frame plus radiator,is it front or rear steer and still have room to fit everything in
The number one thing you have to ask is are you capable,do you have the time and experience, anything is possible or it might be less work to look at after market suspension ,hope this helps in some way.
Check out magazines like "Rod and Custom" and "Custom Rodder". Some companies make new replacement frames (updated) for popular models. There may be one for the 40 Ford.
Zonk covered about everything. Unless someone does make a chassis for your car it can be a lot of work. I'd look at all the small trucks -- Toyota, Nissan, Mazda, and of course Ranger (1990?? and later Rangers are the same as Mazda anyway!). You'll still have to modify for engine mounts, but can get the universal type weld in crossmembers to mount a V-8. I don't think shortening or lengthening a frame is all that much trouble, or changing the width +/- an inch or three (steering has to be changed the same, and steering box location could be a problem), but I have some structural welding experience. If you have to pay for everything to be done it's another story! If you're a decent welder that's fine, or have a friend that can weld and you can help with other projects.
Check the local clubs too. You could host a "fun build day" and set up a bar-b-que and beer and get some "free" work done! I've seen clubs get together and bash most of a car together in a good day. Be warned though, even if you don't have any greatly demanded skills, the guys WILL expect you to turn out for them when they've helped on your car. Otherwise you'll get turned out QUICK!! You'll probably have to help on a couple projects befroe you get a lot of help, but it's worth the time investment.
I have to ask why you feel the need to replace the frame? Is it damaged from an accident or has a previous owner hacked it up? If not you might consider keeping the frame and just adding a decent IFS system and new rear suspension. 40-48 Chevys had fully boxed "top hat" frames that were very strong, especially when compared to the Ford frames of the era. Your stock frame is more than capable of handling in excess of 500 hp if its in decent condition.
I would reassess your thinking about replacing the frame unless it has been damaged beyond repair. The amount of work required to swap frames can be almost overwhelming to a novice and the amount of fabrication required to get the front sheet metal on correctly is not trivial.
If the frame has been destroyed you might consider an S-10 long bed frame which should be close to your wheel base. Again, there's a lot of work involved so don't do this unless you have to.