sounds like you must have a real long nosed water pump,
my 283 mounted on the factory front pegs has 4+" rad-to-fan, and that's with a 2" spacer on the pump .... you could find a different water pump easier than moving the engine, and if it really is mounted as factory, you'll find that moving it back 2" results in interference at the firewall, especially if it has a fat HEI
why a flex fan? not cooling well? 1.5" won't cool well without a shroud, the clearance needed without one is about 1/2" .... or maybe your rad has been misplaced?
Bill
