The spacer needs to be approximately 1/2" thick. Contact brad@ipsengarage.com
Just lengthen the down pipe on that side. That's what the muffler shops did.
If you're reusing a good exhaust system, or bought an out of the box solution from someone like Waldron's, extending the downpipe may not be an option.
If you have the heat riser valve, I would go down to your local machine shop with that, and they should be able to mill something for you in the same dimensions as the heat riser without a problem.
Eight years ago I had a machine shop make me one for my '54 for $80. Last week I went to a smaller machine shop and had one machined for my '61 for $60. Both CNC milled and took the old busted ones to the shops for templates. T. Irvin
Have one water jetted, then drill the two holes.
Art Garnder used to sell those ready made. Contact him as he may have one left. Other option is have a machine shop make one as others have said and which I had done for my '49 20 years ago. Or, if you have an old heat riser (manifold control) valve that is not cracked, remove the valve and its axle, then weld closed the axle holes to make a spacer.