Nothing is screwed up. The engine is out now so do a good clean up of whatever is dangling from the firewall, clean and paint the frame in front of the firewall, maybe overhaul that front suspension and possibly run some new wiring - some of that original wiring wasn't much from the factory. Then there is the change from 6V positive ground to 12V negative ground you will need to make regardless of what manufacturer's engine you use. Now, you need motor mounts bought or fabbed then welded in, a transmission mount, again, purchased or fabbed installed after you decide on what you are going to use - that original transmission, I can assure you will NOT survive even the lowest Hp 302 unless you drive like my granny, and then it's problematical. You will need a Fox body rear sump oil pan and oil pump pickup to clear the front cross member plus may need to 'drop' the drag link regardless. The rest is standard, fab work on exhaust, linkages plus gauge mods for the negative ground. Yeah, you can put a Chebbie in, but so does everyone else and it's not a bit easier with exactly the same work needed to do the job.
Do it, ask questions here and elsewhere, but before you start, and as evidently a newbie, draw up a basic plan as to what is needed and a general sequence to work to. Replacing a flathead IMO, is almost always a good idea these days unless you are really wealthy as it takes a bunch of bucks to even reach what a run of the mill 200-250Hp 302 does in stock form. If money is no object, then a fully dressed flathead - nothing sounds as good or looks and nice in a '49-'53 Ford.
Dave W