Ok I looked into a few things awhile back, both WVO conversion and long range tanks.
I found out that a troopy tank would not fit under a 77 because the 77 does not have enough room for the length of the tank. A cross member is in the way.
There can be a long range tank bought out of Australia that is 170-180L in capacity. The new cruisers are coming standard with 130L tanks overseas, well at least in the 76 series. You will need to cut the storage
pocket out in the floor to make them fit. I don't see this being a big deal if the cruiser has drawers in the back, because you wouldn't be able to access anything in it anyways.
Simon Liew did an aux tank for WVO on his old 77. Take a look at this thread.
https://forum.ih8mud.com/70-series-tech/472637-my-1992-hzj77-built-turbo-intercooler-vegieoil-2.html
I am sure something could be made easily, I think it would be a matter on how you have your exhaust routed to give yourself room for a second tank. You could mess around with cardboard and make a rough tank yourself. Take the dimensions to a competent fabricator and pump one out in a few hours.
Make sure if you do a WVO conversion that your fuel is de-watered and clean. Extensive heating and filtration (centrifuging) is best. Water test all of your fuel before putting it in the tank. Also install a water catch in your system with a glass bowl (Not plastic). Make 100% sure that the oil you are putting through your pump and injectors is above 70 Degree Celsius. This is the point where VO is the same viscosity as diesel fuel.
Keep us posted what you do, there aren't too many 77 guys around here. Did your sweet 73 sell?