I don't know if you can simply machine the J4R block to J4RS spec, if not you could still create some external plumbing like on the Sierra Cosworth engine.
If you don't have a XU10J4R yet then looking for a XU10J4 engine is not a bad idea, the head is the same than the 405 T16 minus the cams and sodium filled valves so you know the design is fine with boost. In fact it is based on the 205 turbo 16 evo head and was also used on the 206 WRC by taking advantage of homologation rules.