Yes the blocks are the same externally as any 289/302. It's true they are weaker than your old 302(depending how old it actually is). In fact a late 5.0 block is 24lbs lighter than the old 302 block.
The factory engineering rating is only 350hp for the late block and 500hp for the old one.
It sounds like you're after a mild combo so the late block will do you fine.
The conversion is fairly straight forward(Ive done an XK ute and an XP delivery). There can be clearance problems with the serpentine drive accessories. Less desirable but otherwise an option is to switch to the old V-belt timing cover/pump assembly that you already have. It's a good idea to move the battery to the boot to free up room for the MAF/Air cleaner assy.
It is a very worthwhile conversion.
Check out my old XR conversion. It was a 351W but most of it still applies to yours.
http://www.trueblueford.com/66'XRfalconEFI351wconversion.htm
Pete.