The 73 runs all the 75 running gear. The difference in engine is only with diesel. they run a 3.5litre 3b diesel instead of the 2h that 75s run.
The bundera runs all hilux running gear with a 24 engine.
The tow bar you ordered will fit as 73, 70 bundy and 70 leaf sprung are the same width and...
You said that your midi comes with the offset tank under the middle. Mine comes with a factory tank at the rear. As was said further up this post, some cruisers come with duel tanks from factory.
Imo if you love the midi I would put a second tank at the back. I can get 650 klms out of this...
I bought a midi that was a bj73 with the 3 seat setup. The tub was rusty so I bought a tub that was a bj74 tub with suspension bucket seat setup. I had to swap the seats I had for the suspension bucket seats. there was no way that they were going to interchange without some serious mod work to...
What I am going to do to mine fairly soon is swap the whole tub over on to a 80 series chassis. I know someone who did it with a 75 series (ute) Basicaly all he had to do is move cab mounts and shorten the steering rod.
On a midi you also need to cut a bit out of the back of the rear wheel...