There were many differences between a small block and a big block van. I saw a list once on another website. The hole in the floor for the engine is to small. The engine cover is to small. It seems like some parts on the engine were also different. (Front cover perhaps and ????) I can't recall all the details, but it was a LONG list!

It would be easier to install a 360 based stroker engine. If you went this route, magnum heads and a 518 overdrive transmission should make a nice combination.