89 HJ61 12HT auto.

[V6 Bundy]

That tank basically fits under the central rear passengers feet/arse... inside the frame rail, ahead of the rear axle and beside the tailshaft obviously..... I have one in a 2h powered 60.... there should also be a couple of other brackets... I'd have to have a look to be more specific..... the PO on mine cut a hole through the left rear guard and added another fuel cap there, but it's as ugly as a hat ful of arseholes with the good ones picked out... just a round generic cap sticking well proud of the guard. If you're any good at panel work maybe a second original cap/flap etc in the left rear quarter or a dual throat thing under the existing cap.... I do believe it makes for a fairly slow fill on the LR tank though if you're coming from the factory filler position.....

If you have the chance, I'd love to see a pic or two of you tank installed. I had a look underneath where you were talking about but I just can't get my head around the mounting positions. None of the existing bolt holes seem to be in the right positions.
It did come with a couple of 'U' bolts and a piece of rectangle section, so I assume that crosses the chassis rails and the 'U' bolts are used around that.

 

[V6 Bundy]

The angled part of the tank looks like it goes parallel to the diff and the rectangle cut out must be where the relocated exhaust goes.
If that's the case, the filler lines etc would be directly facing the driveshaft. That doesn't seem right?

 

[Barney60]

As stated the tank fits up under the belly of the cruiser, between the rear drive shaft and the exhaust. The angle part is clearance for the diff centre. It looks similar to a 'Long Ranger' tank but i don't think it is one. The rear of the tank mounts to the rear round crossmember and the front should mount to a bar which mounts to the chassis. It's also worth checking if it will fit because there is a 100mm differance between auto and manual tanks. If you have one to suit a manual you will never get it to fit an auto as it will be too long (happened to me). The subtank filler T's off the main tank filler and is filled this way. There are alot of parts needed to fit a subtank and it gets rather difficult to fit one without them.