First the 750 Ebrock is a performer series carb , the thunder series is only 650 and 800 , the 750 has issues tell your buddy to get a Holley.
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some of the oem type replacement pistons are around 8:1 compression with 88cc heads and .039 head gaskets. going from 10:1 to 8:1 will kill a bunch of torque off. from looking at the 750 edelbrock specs they seem to be calibrated fairly rich. i'd also look at the advance curve also, especially if the compression is down.
As far as the 383 goes there was NEVER a real 10.0 engine out of the factroy in 68/69, the compression is actually more like 9.3 at best and going to 8.0 would make it worse .
