I'm pretty sure pumps made for a 350 will fit the engine. It's the asessories that brackets and the fan might not fit at all or well.
If you are replacing a 350 with a 350 just ask for the pump to the vehicle it fits in. Do you still have the old waterpump? If not then why? If this is to be a totally custom job you will be scrambling for the parts to make alt, ps, A/C and whatever fit properly,
T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/