Just my experience - I had been bracket racing my '67 Shelby for about 12 years with a set of old Motor Wheel Spyder aftermarket wheels, probably about some of the heaviest wheels, with a heavy cast aluminum center and a chrome steel rim, and then switched in the mid-'90's to a set of Bogart "Drag-on-Fly" Stars, spun aluminum wheels very similar to the old Motor Wheel Flys, and some of the lightest drag race wheels available at the time. Car ran consistent low 12's, about 3450 lbs with me in it at the time. Same size rear wheels, 15x8" with the same 28x9" Good Year slicks. Fronts changed from 14x6" Spyders with bias ply street tires, to 15x4.5" Bogarts with Good Year Front runners. I did weigh the wheels at the time, but the info has long since been filed away somewhere, but the difference was significant. I needed to change to longer wheel studs for the Bogarts and the only company making the special studs for the early Mustang front discs was ARP, and they were out of stock for a short while, so I wasn't able to change both front & backs at the same time, and the change was made part way through the race season at the time, so I had some good data going. I put the new rear Bogart wheels on, and the difference was noticeable immediately on the first burnout, the rpms spun up much quicker, and the car ran consistently one tenth ( .10 ) quicker. By the next race I had the new front Bogarts and lighter front runners on, and saw very little change in ET, nothing measurably different that weekend. So that's what I found, the lighter rears made the biggest change. You're not making as much of a weight change as I did, your current wheels will be a bit heavier than the Bogarts, and the Cragar S/S's are likely a bit lighter than the Motor Wheel Spyders, So my guess would be you will slow some, maybe 5 hundredths (.050)?