For ITS a 240 is probably best. A 280 will make more power, but has to run under a 300 lb weight disadvantage.
For the street get a 280. The price of clean, rust free, 240 has increased substantially in the last couple of years, a 280 in the same shape is still relatively easy to find. Also, a 280 has a simple fuel injection system, easy to diagnose problems and repair. A 240 has dual carbs, which I have always found to be troublesome.
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Ty