Well I went ahead and did it, so I thought I'd better share!
I cut off the old big fuel pipe from the MGB gas tank and filled in the hole, as well as the small hole near it. (1970's MGBs here had a return fuel line to send fuel vapours to the engine bay) I then cut a new hole as close to the corner as I could and welded in some exhaust pipe of the same diameter as the original HB filler pipe, trying to get the angle about right. I cut the filler pipe off the rusted out HB tank and have a short bit of rubber hose between it and the new tank's pipe. I also cut a circle of metal to fill the hole where the MGB sender was on the side using a new rubber seal. I would have just left the old sender in but I threw it out a year ago as it didn't work. Silly me, should never throw anything away...
Rather than buying a new MGB fuel sender and a converter, I found this Chinese one on eBay that will work directly with the original gauge. (0-30Ω)
https://www.ebay.ca/itm/193843366021?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2648&autorefresh=trueIt's a 6.5" vertical shaft with a ring that floats up and down along it so no long arm in the tank, which is nice because 1970's MGB tanks have a surprising amount of things to avoid inside. (picture of cut open tank is from MG Experience forum) With this knowledge of the innards, I drilled holes to mount it between the baffle and the huge round thing the vent line went to. It just touches the bottom of the tank, a perfect fit!
The tank fits nicely, bolted up between the frame members, needed spacers on the bolts holding it up as the flange isn't flush with the floor. I also cut a metal plate for the floor of the trunk over the tank.
The tank's has a screw on fitting for the fuel line. (on the side above the old sender) I reused the MGB line and bent it so it goes neatly around the tank to near where the existing fuel line to the engine bay is. In fact, I put an electric fuel pump there as my mechanical one doesn't work. The fuel sender is earthed along with the fuel pump using one of the bolts I mounted the pump with.
The only modifications to the body were the holes I drilled for the mounting bolts, so if I ever happen upon a good Viva/Epic fuel tank, I could easily remove the MGB tank and drop the original style one in.