You need four inches in front of the existing 4.3 to slide in a 5.0 or 5,7, also side mounts on engine block will be 4” farther forward as well. I did the same swap 4.3-5.0 on my boat but the boat was offered with a 5.0 from the factory so the engine compartment was big enough and the stringer mounts were 12” long To accommodate either engine configuration
would definitely measure before picking up an engine as it would be a lot of work to stretch the engine house in some applications like mine where the bench seat is in front of the engine, stringer mounts can be glasses in fairly easy though