Unless you really want to inflict a lot of pain and agony on yourself, just replace the four banger with another one just like it.....or sell it off as is and get exactly what you want.
No the transmissions will not interchange
To successfully pull off a 2.5/4.0 swap in an 03, you will need a like year (or very very close to it) as a donor. You will need it pretty much whole sitting side by side.....or refer to the first paragraph. Been there, done it, won't do it again on an 03