My 1973 337G has extended-range tanks but of the 128-gallon variety. However, it's likely that the 148-gallon version has all or most of the added capacity outboard of the fill hole, in which case you're likely to find that the "just-see" amount is around 30 gallons.
If it looks level, it's good enough. Also, if you use a carpenter's level (with a bubble), there should be some park angle where the two main wheels are level. Perhaps more important is that both wings have similar amounts when you do the fills to the "just-see" amount. When I return from the island I have used about 14 of the 30 gallons I had on the island, so both tanks have about 16 gallons, and that's close enough. A situation where the opposite wing is entirely empty or completely full will, of course, change things.
Ernie
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