I would get a GN / 4.3 Monte tank since they were baffeled for the FI
90's Caprices had a 22 gallon tank
Will it fit? I have no clue.
If your main concern if fuel economy then I would invest my time and money into an overdrive transmission.
if your looking for only another 30-50 miles from your tank, the improved fuel economy associated with fuel injection may get you there with out a larger tank.
I guess he will have to build up that BBC more to pull the extra weight.
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