I have two 200 gallon saddle tanks that are in the process of being replaced with four 100 gallon fiberglass tanks. I considered aluminum, but ended up with FG as aluminum was considerably more expensive, and I like FG for tanks.
Single Perkins 4.236 (75hp) diesel engine was out in under a day. Other work being done so engine isn't back in, but probably 1-1/2 days to reinstall with alignment.
Having the work done in Mexico. Turns out Ensenada, jumping off point for the Baja 1000 off road race has decent fiberglass workers due to custom truck /car industry. Not including fittings or engine removal, total cost including install and removal/disposal of old tanks was $6500. Fittings were about $1200 when done as I went with beefy fiberglass clean outs made for oil field fracking usage - they are glassed in and have no fastener penetrations into the tank.
When splitting two tanks into four, there are two options for fuel pickups. One is to have four pickups (and returns) - one for each tank, though this increases the valve complexity by quite a bit. Second, you can combine each pair with crossover valves at the tanks low point, which is what I did. I'm told that crossovers meet ABYC but should be protected from accidental impact.