Ok, first thing's first.
As far as the radiator leaking, new or even used radiators can be had for dirt cheap. Get that taken car of as soon as you can, though I would say the engine and transmission deserve more immediate attention.
As far as the transmission goes, as far as I understand things, if an automatic has not has its fluid changed in xx,xxx miles (I believe it's 75-100K), then it may not be the best idea to change it. The way it was explained to me was that the fluid after xx,xxx miles becomes something of a varnish on the internal working parts of your transmission. Changing the fluid after this has happened causes the varnish to come loose and can damage internal transmission parts. I've personally never had this happen, anyone able to back this up? Again, this is just what I heard. Most oil/lube shops won't change your tranny fluid after those miles if it hasn't been done before.
As a couple others have said, the 5.0 can take a ton of abuse (Ford's best engine, IMHO). 70,000 sounds REALLY low for a rebuild, I personally have about 110K and the engine is the last thing I worry about. If you were slack with maintenance, then *maybe*. My bet is with your transmission.
If you need to have it rebuilt for whatever reason, I have seen rebuild kits in the area of $300-$1300 depending on the metals used and whatnot. And labor is what will probably drain your wallet the most. For a straight-up rebuild, I would guess $500-$1500 depending on the quality of the shop. Get an estimate ahead of time, as always.