The pump mounting parts are the same for the '67 GS400 and the '70 GS455 according to Buick parts books.
A cast aluminum bracket part no
1378150 attaches to the engine. This part is used on Buick '67 A body except with AIR pump, '68-70 A body V8, and '67-68 LeSabre.
Stamped steel brackets (called adapters by Buick) attach to the aluminum bracket and the pump. Front bracket is a
1378368. Rear is
1378369. These are used on '67-70 GS400-455 and '68-69 SportWagon 400. Different adapters are used on the V6 and small block V8.
The A body parts book printed in 1970 shows the pulley is a
1384927 for '67-68 cars (except L6). The '69-70 cars (except L6) use part no
1386935. The later Master Parts book printed in 1972 shows the
1386935 for all '67-70 Buicks (except L6). Examples found on the Internet have the part number stamped in the pulley.
The pump for '70 Buick A body V8 cars is a 7807378. Otherwise the '67-69 cars that use the aluminum bracket have a pump no 5698071. The difference in the pumps is thought to be minor since the brackets used are the same.
See attached drawings on the belts and power steering components from the Master Parts book.