I recently changed an existing RV-8 tank to a flop tube, this involved:
-cutting a new access hole in the back tank bulkhead for new float location, install reinforcement ring & nutplates to mount float cover plate.
-Float assy & cover plate unchanged other than plugging off old fuel pickup hole
-installing the slosh back valve in the bottom corner of #2 tank rib
-installing a vertical fence bracket to restrict the flop tube sideways motion
-installing the flop tube per plans, incorporating an anti-rotation bracket to keep the 90 degree bulkhead fitting stable
-fabricating a new cover plate for where the stock pickup & float sender used to be
Only thing I did different from plans was to install float sender unit in the outter half of the first bay, being sure to be clear of any flop tube motion (mounting in first bay made easier access for working & cleanup of drilling debris & sealing, etc.)
Note- the location of your tank vent line & insure it does not interfere with flop tube vertical motion!
Maintain lots, upgraded - repaired - & modified more, rebuild a few, & built some too.