The '72 Buick master parts book shows the complete pump to be part no 5694686 for '65-'66 Wildcat/Electra/Riviera cars. The Special/Skylark cars used part no 5694723 in '65 and 5696606 in '66.
The pump shaft is a 5694979 for all '65-'66 Buicks. The Buick Farm has a rebuilt 5694686/5694723 pump insert available with a 1/2" dia shaft. Some other vendors show a 9/16" shaft which appears to be too large for '65-'66 Buicks. Unless machining was done on a lathe, attempting to cut down the shaft diameter would likely introduce runout, wobble, and a loose pulley fit.
The pump housing (casting that pulls out of the reservoir) is a 5688072 for all '60-'66 Buicks exc 61-63 Specials. Various Olds and Pontiacs also used this pump housing number, but with different shafts. The housing casting number shows as 5688074 in all the rebuilt/original photos I could find. There seems to be no need to use a different pump housing having the large diameter casting extension surrounding the bearing. That just makes extra work to enlarge the bracket hole.
The ACDelco website shows their 36P0286 rebuilt pump insert fits the '66 Riviera. Another ACDelco number for it in Canada is 19318812. So that might an avenue to find it there without importing. Rock Auto and Summit Racing show it available just in case.
Problems with squealing have turned out to be loose, worn, or glazed belts from my experience.