I've done this on my 53 Pontiac and my 40 Buick, using m/cyls listed for Dodge Caravans up to 1995. Rock Auto has them for $35.45 Canadian, so US price would be significantly less. I used a Caprice proportioner valve on the Buick. I did the Pontiac one long ago and don't remember what proportioner valve I used on it. The Pontiac mod has worked fine for several years, including a long trip towing a camper. The Buick project isn't ready for the road yet, so I can't report on it. A bit of bracket fab, a bit of plumbing, and "Hey presto!" I think the push rod has a straight run to the m/cyl on the Pontiac, which may be set up like your Buick.