Also, L&L makes an engine mount kit for $175. Looks like it is of pretty good quality. I would consider ketting this...
Just FYI those motor mounts bolt straight to the engine cross member, they do not use the stock motor mounts. On first (and maybe second) gens they bolt directly into the hole used by the 2.8 & 2.9, on later trucks you have to drill holes for them. They look like solid mounts but they aren't, they have a rubber spacer made into the top half. It's hard to tell in the pics but when you see them in person you can see it. I have a set waiting to go into my 86. I can say that they look nice but the 4.0L mounts are a better value. Only reason I have these is I picked them up for $100 locally from someone that was planning a swap them selves, they got rid of the truck and had no need for the mounts.
...but I just read that the older Econoline tanks fit where the spare tire is mounted. For me that's jolly good news for three reasons: it frees up the space in front of the axle for IRS supports and exhaust, it moves all of the weight towards the rear of the truck for better weight distribution, and it ups the capacity by roughly 5.5 gallons or more.
Never read that the Econoline would fit. You might want to do some measuring on that. It is about 1 1/2" shallower, but it's also larger in length and width. I'm not sure that it would fit between your axle and rear cross member. Due to the shape it's also not going to tuck up into the frame as well as the BII tank, at least not without making a custom rear cross member. The BII tank is conformed to the shape of the rear cross member (BII and Ranger have same one) it tucks up into the frame a lot nicer. You can also cut one of the rear cross braces under the bed floor and the tank will go up into the frame at least an inch more.
For reference the posted dimensions on each tank are as follows:
BII: 29-1/4 in. L x 24-5/8 in. W x 13-7/8 in. D
Econoline: 33-5/8 in. L x 23-1/2 in. W x 11-1/4 in. D
As I said, unlike the BII tank, the Econoline tank isn't shaped to fit against the rear cross member. When measuring for it you would need to measure from the front of the rear cross member, or figure out where you would need to make your custom one.