Doesn't make any difference in fitting, but the 83 engine has hydraulic valve train, where the 70 engine is solid mechanical valve train. Also the cyl head takes different spark plugs and a different valve cover. The intake manifold might need to be changed as (and I am not sure) the 83 engine might have a 2 bbl carb, while the 70 engine will have a 1 bbl. These are just some things to be aware of and will not affect the engine fitting in the van.
PS: before getting into an engine swap, check and clean the oil pressure relief valve in the oil pump. It might be stuck. It is external to the engine, and easy to do.