Mickey - A 1961 Chevrolet Master Parts book shows the spacer as GM# 3721152 (no dimensions given) as shown in first pic below. A more detailed part description found in a 1963 GMC Master Parts book showing the same GM# 3721152 gives the following dimensions:
Spacer - 2" wide x 4" long x 1/2" thick with 1 each 33/64" diameter hole in center.
If you can't find an original, those specifications should be enough to fabricate a spacer to fill your need. You will need new (and possibly longer) spring to axle u-bolts to install. Best of luck in your hunt!
