External dimensions are exactly the same.  The recipricating assembly, and
balance is differant.  So you need the short block, balancer, and flex plate to match
the 360.

While the engine will fit, there are some differences.  The two most
important ones are the computer and the fact that the 360 is externally
balanced while the 318 is internally balanced.  The fuel curves are
different for the 360 and the 318 computer may not give you satisfactory
performance and I'm not sure about the legal situation of swapping the
computer from a different vehicle.  As for the balancing, IIRC you will need
a torque converter balanced for the 360.

I am betting you will find what you need here. http://www.hughesengines.com/ 

Thank you all for your input... I'll have to let you guys know how i
ends up. Looks like I will go with the 360 5.9L Long Block, no rea
need for the performance mods. I just want more power to tow a trailer
especially when travelling uphills.

My 318 has about 200k miles on it and the rear seal is starting to lea
so it needs to be pulled anyways, hence the question of upgrading to th
360.

Thank

--
plaidfr

Hi,
I have successfully done this. You have a couple of concerns. The 360 is
externally balanced, so the flex plate will have to have to have the bob
weight. In the case of a manual gear, you will need to use a 360 flywheel or
mill the corresponding volume of iron off the opposite side of the flywheel
that the bob weight is on the flexplate. I did this and it just worked.
Depending on the year, you may have to drill and tap a 3/8 NPT thread in the
top of the inlet manifold to accomodate a thermal sensor. You may use the
5.2 computer with the 5.9, but you will have to change the injectors to the
5.9 flavor. In my instance, I had to change the connectors on the injector
harness as they are also different. This is a very gratifying change. You
will have lots more low end torque, more high end power and better economy
by 2 MPG. Work that out!
Steve