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There was a bunch of casting flashing at the location the arrows point to.
I measured the smallest x-sectional ID before I ground it and afterwards and calculated a 13% increase in cross sectional area. To the extent it was formerly restrictive, it's less restrictive now. I'm running a 160 stat. When its 85 deg outside my crate operates 169-171 and yesterday when it was 101 deg outside it ran @ 174. I'm using the Meziere. Another area are the bends in the block where the coolant enters from the pump - horrible bends with a big slug of iron in the way. When I did the pump swap on my 93 last week I stuck my fingers in those holes and felt the blockage. On the bare block I'm working on, I made niceties out of those bends. Nice smooth curves w/no impedance now.
If you go to the Meziere HD electric pump you never have to worry about it leaking and destroying the opti. I have had mine on for two years with no problems. I carry a spare relay and an emergency jumper to just by pass and direct wire it in case of emergency but have never had to worry about it.
I have a stock water pump on my 383 and track the car with no cooling problem. I'm not a big fan of the EWP. I don't see any reason to upgrade from the stock one, it works well (when it's not leaking).