I also have a 64 chevelle that I'm starting to buy parts for. Like you I'm planning a small block overdrive combo. I'm going to by a zz4 long block and swap the cam out. I've been looking at the XR276 from Comp Cams. Looks like a great cam for the 1900-5600 range. This would be my pick if you wanted a hydro roller. As for the rear gear I would run a 3.73 with the overdrive transmission. Call Mark at Bowler Transmissions for a 2004r, he's a great person to deal with and knows his stuff.
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
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