I've got a 63 dart with a magnum headed 360 la engine. Everyone is right. Almost everything needs to be changed to make it right. As far as the firewall.....I just had to reshape it a little with a BFH. Just make sure you remove the wiper motor first. I tried beating on the firewall with the wiper motor in and snapped all three of the mounting legs. I got a new wiper motor and the wipers still work even aftr the reshaping. If you need any help at all I would be more than happy to help. I was just like you, I bought my 63 with no understanding of the hurdles to get a v-8 in it. About 1 year and tons of questions on this board got it done
Good luck.
I almost forgot.....you will need some extra cash too