RB'sObsolete Automotive sells conversion motor mounts for a 350 in a 53 Chevy. www.rbsauto.com. They specialize in old Chevies and are the best source for hot rod parts for old Chevies.
If you use the Mustang front end I think you should use the heavy duty springs that mike suggested. The weight difference between the stock engine and the 350 is not the issue. It's the difference between the Mustang V8 and the 350 that needs to be considered. The front sheetmetal on the Chevy also weighs more than the Stang. The regular Mustang springs were for a four cylinder engine.
Check out the Pontiac Fiero front end. The front crossmember can be installed without all the welding and fabricating. I don't know if the springs are stiff enough for the 53 but a spring shop can twist up a pair of heavier springs for you if needed.
The 54 Chevy tail lights are a bolt on swap and are much better looking than the 53 lights. Extra grille teeth are all you need to make the front look like a custom car.
AL