[Steve Barcak]

It can be done. I built a '59 389 with an even larger stroke at 4.800". However, I also raised the deck 2" to run a much longer rod and built an extremely heavy duty 6 bolt main lower end where all 5 mains were tied together. It is quite elaborate compared to anything else I have seen in the Pontiac world. this was done before aftermarket blocks were available.
When you increase the stroke, you do lower your torque curve but, also reduce your rod to stroke ratio. This adds side load on the block and lowers rpm level.
If you wanted to build a street motor with this much 'arm' as they call it, you could. If the power level was modest and the rpm was kept down, you would probably be OK. Building this type of motor is known as a 'hand grenade' because of the stress level placed on it. Definately make sure the block in in good shape.
However, I think you could be plenty happywith a 4.25 or 4.5" arm and a slighly better r/s ratio. You will also have to consider your chamber size on your heads.