Matt: I have a set of the P5 "wedge" heads, and checked on the R4 blocks. As far as I know they are 8.8" deck height without lifterbore machining which are the reasons I did not buy this block. I decided to go another route; use an alu sprintcar block, and change the lifter galley (involves a lot of fabrication and welding). To stroke it spacers would do, and longer sleeves is no issue. But the problem is the headbolt pattern is also different, so I just put these heads on the shelf, maybe it can be done in my next life!! (LOL) Also checked them on R5 block, did not fit there either....

If you do this and add spacers you should do something with them to get coolant flow high enough to take away heat from the pistonrings at top dead center.
It will be a costly project unless you do the machining yourself.