Not sure on a 261 but a GMC you can make the center hole on a BBC the correct size then drill 6 new holes to match the crankshaft and it will work.
At Bonneville you will need either a steel or non-cast aluminum. What ever you find I would add 3 5/16" or 3/8" dowels...(36 years on the salt with inlines).
Check with Clifford Research...Not cheap but what is now days.....Good Luck

216.158 MPH 12-Port 302 GMC on 70% 171.0 MPH 302 stock head on gasoline 7 years later