I think the best and cheapest way to begin would be to get a donnor car such as an XD or XE V8 (or even XA - XC). You could then source most of the drivetrain (engine, trans, trans crossmember shifter etc,) from the donnor car. However XE V8's are extremenly rare and 351's were only avilable in the ESP's, Fairmont Ghias and LTD's. The beaty of the clevland engine though, is that you can get a 302 and have it stroked to 351 when you rebuild the engine. There is a place called COME Performance that provide stroker kits for these engines up to about 378ci.
Puting a V8 into an XE is a pretty easy process, even if the car was originaly as 6, all you need to do is install V8 engine mounts and a new crossmember for the trans.
I am not aware of a C5, howver an FMX from a 351 XD will fit in the XE without modification. In most cases though a good C4 will do the job.
If the car was originaly a V8 then the standard diff should be OK, if not you can use the standard 6 diff housing and install 4 pinion LSD and 28 spline axles.
Hope this info helps, best of luck with your project.