According to my book the 55Chevy was the first Chevy to offer the Power-Pak
The 245hp and 270 hp engines were 2x4 engines.
The 250hp and 283hp engines were FI engines.
The higher output versions had the solid lifter camshaft, commonly referred to by all knowledgeable Chevy folks as the "097" cam (last three numbers of the part number).
The lower power versions had the stock hyd cam.
ONLY the FI engines got domed (or commonly called pop-up) pistons. all other engines had flat top pistons.
ALLLLLLL heads had 1.72/1.5 valves.
In the 57 pass cars, there were a total of seven V8 engines:
265 2bl
283 2bl
283 4bl (commonly known as the power pac engine)
283 2x4 (hyd cam)
283 2x4 (solid cam)
283 FI (hyd cam)
283 FI (solid cam).
The 57 Vettes DID NOT get the 265 or the 283 2bl.
The top rated 57 283 was 283hp with FI and the 097 cam. BUUUUUUUUUUUUUUT, during CEC (Chevrolet Engineering Center) tests, the 283/283 FI engine developed a true 293hp!!!! This rating was downplayed SPECIFICALLY for advertising/promotional purposes so that Chevy could tout One Horsepower per Cubic Inch! the 58-59 solid lifter FI engines (which were virtually identical to the 57 engine) were rated at 290hp.

