OK Classic Industries has this part;
Pontiac Engine Conversion Brackets-Late Eng. Into 1st Gen.
This kit facilitates the use of a 1970-81 Pontiac engine in 1967-69 Pontiac F-Body chasis. Due to a 3-bolt mounting configuration on 1970-81 Pontiac engine blocks, an adapter bracket becomes necessary for proper engine positioning. This 2 piece conversion bracket set converts 3-hole motor mount pad on a block to 2-hole mounting.
C269514 70-81 engine into 67-69 Pontiac F-Body $69.95 pr.

That is verbatim from the catalog. I've run into a few of these 3-hole blocks. I've also been lucky enough to find some that had 5 holes to accomodate either style mount. You could make your own brackets but it take some time and measuring if you haven't done it before.