That truck most likely came from the factory with a six in it but I'd have to think that a large percentage did.
There are several ways you could go on the front mounts including getting the original style brackets, using one of the universal under the engine mounts or fabbing your own mounts.
It doesn't absolutely have to have the rear mount on the trans if you use the bellhousing mounts as stock tri 5 trucks don't but with that aluminum case and the length I'd think that a mount at the back of the trans would give a bit of additional support and security. Again one of the universal aftermarket crossmembers would work or again a guy could fab something.
On the side mounts that are in it, someone may have had a V8 in it with a Hurst style mount that is often used to put a small block Chev in early V8 Fords. The spacing looks pretty close for that.
Here is a link that shows a Hurst mount
http://www.oilnspeed.com/albums/v/sp...02mod.jpg.html You have to use your imagination a tad to see how it works.