It's the Edelbrock 500 EPS (500 CFM, Edelbrock Performer Series) Best I recall in the owners manual for my 600 (it's on a bent 8) it calls the only difference between the two (in the air flow path) is the booster cluster on the primary side of the carb. Having seen both, the 500's boosters definetly appear to be larger and I would expect a larger vacuum signal fom them (seems a mite excesive to drop 100 cfm that way, but they manufacure it).
You can download the manual for free here:
http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdfIn it are specifications which incled the bore sizes of each carb, what the stock jetting is (was) and a graph the tells you which jets / springs / rods you need to chage to get a desired effect.
For 600 CFM you are looking at the 1405/1406 that's manual/electric choke respectivly (calibrated for performance/economy respectivly).
I can't seem to lay my hands uppon my Nov/Dec '08 issue at the moment (Sept/Oct '08, but not the next one).
600 seems kind of large for a 250, if you are driving it on the street. Of course you could probably make the case to match the carb to the manifold, ie a 600 with the Clifford and a 500 with the Offy (maybe, just speculating there so it's a guess on my part).
Hope something in there helps.
