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Question; is it possible to use the original fuel tank from a 1976 car in combination with the slightly taller 5-speed back axle,
or do a need a later tank with the recess in the bottom to get enough clearance between them when the suspension bottoms out?

I had no idea the early and late carb' tanks were different as I've only seen one part number in my BL parts book.

They are, this is the under side of an early tank ...



and this is a later tank (in this case a FI, but the later carburettor version also have this recess) ...

Which matches the recess on the rotted tank I removed from my 1980 DHC and it's replacement...... I had no idea the early tanks had a flat floor.

Not a problem Beans, let the diff make it's own way, [space], in the world.

Of course, you could just grab the large ballpeen hammer, & "adjust" the tank yourself.

That's what we had to do to the sump of the 8, to stop No.1 big end conrod bolt of the 4.6L forged conrod hitting it. Annoyingly we did not go far enough the first time, & had to pull the sump off a second time & really whack it.

Hasbeen

The steel is thin and after 40 odd years may have reduced further. "Massaging" the area to replicate the original pressing on tanks may be too much if there's inherent corrosion.

To demonstrate how corrosion can take hold from the inside


It's just light surface rust on the outside. The true rot was on the inside.

Adam

I think Beans is talking about a new tank, not a patched one.

I welded a 6" wide strip across the bottom front of my original 7 tank about 14 years ago. The inside rust was only in a strip that wide where any water would lay. The rest of the tank inside still had it's original surface coating in good order.

I replaced that tank with a new one a couple of years back.

Hasbeen

No, it's not new but should be pretty solid, it was actually rather mint on the inside when I pulled it from the car.
But some 3 years in storage has resulted in a fair amount of surface rust on the inside.
Looks like I'll have to visit a local supplier to get me some Tank Cure

There's always something new to learn, I thought they all had the 'dent' in the underside. But then I've never owned an early 4 speed car.