As someone who spent several thousand dollars rebuilding their 72 transporters type 4 a few years ago, I also say subie, depending on what you are doing with the van...
I know your opposed to it, and I was too, hence still having a type 4.
First off, the rebuild nickel and dimed me way past my initial budget for the engine. By the end of it, it felt like every single little part got machined or replaced. Thousands of dollars were spent, and that was for a relatively stock 1.7l rebuild.
It's been a few years, but every time I drive it, there is some level of anxiety about that engine. Moreso than any other engine I've rebuilt or swapped. I have a head temp guage and oil guages, and watch them constantly. I stress at every hill or onramp, just waiting for it to inevitably pop or fail.
At the end of it, I could have spent less money, had a more reliable Subaru engine, and feel less stressed about driving it.
Even if the initial costs are similar, the long term cost and enjoyment of a Subaru swap just seem to make more sense . If a Subaru swapped engine needs service or fails, I can fix it with readily available parts from a FLAPS or grab one from a junkyard and have it in by the end of the weekend. If my type 4 needs service, there is no quick parts availability, and if it fails, nearly all of the money I put into it is gone, and I'm starting the while process over again before it goes back in the bus. With my schedule, that would be weeks at a minum, but likely months.
I appreciate that by bus is oem+ with minimal mods, but I can't help but think I would appreciate the bus more if I could enjoy using it more.