I have had an electric on my '71 for about 15 years now. Its a great conversion as it reduces vapor lock and gets the pump out from under the hot header. I mounted mine on the rear panel just inside from the left taillight.
You will want the lowest pressure rating available, there are usually 2 or 3 to choose from. (I can't remember the exact rating- its been a LONG time..) The Weber on an 850 (or most any other Fiat for that matter) has a very light float and will flood with excessive pressure. Flow rate is not all that critical as most any pump will deliver WAY more fuel than a 903cc motor could possibly use and the 850 has a return line for the excess.
Bayless stocks a block off plate for the old pump which will neaten things up a bit and prevent leaks. I think the block off from a 128 with factory electric pump will work as well. Be sure to install an inline filter BEFORE the pump. On a related note, It would be worth checking the condition of the screen on the fuel pickup in the tank. I have had these collapse and block off fuel flow on both 124s and 850s. If you install the above filter before the pump, you can just remove it and save further aggrevation.
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