I'm not hardly a guru but I do have some serious hours on these clusters.
The speedometer requires no electrical connection - so that will not be an issue.
Do you plan on cutting out the section of the pc board that the left hand gauges are wired to - or just bending it out of the way ?
The fuel tank level gauge should have power, sensing and ground connections (via the RH side of the pc board). There are two groups of power and ground leads on the board. The ones on the RH side (gas gauge) are independent of the gauges on the left side (oil, temp,voltage). So even if you do cut the LH side of the pc board - it will probably still work.
Whether you have dual tanks or just the production tank doesn't matter - all switching is done outside the instrument cluster.
I can't think of a fourth gauge - unless you mean the clock. The clock is not powered/grounded through the pc board - it is connected to the fuse block and the ground bus by external wires.
It would help a lot if you can provide more details on what you are planning to do. For example: Do you plan on installing some aftermarket gauges and run new leads to them? If so, how do you intend to mount them with that section of the bucket cut out?