A few issues to consider.
1) The rear output assembly on all DII cases does not have the hole for the VSS. It is easier to take the rear output assembly from a DI. It will easily mate to a DII case.
2) If you are using the shift linkage from a DI or early RRC with this swap, it is best to also locate the cover for the hi/lo assembly from a DI. It has a lever to which the linkage has a crook missing on the DII assembly. This makes it much easier to set the linkage up. The cover is held in place with six bolts and has the breather on top.
3) Make sure that the DII case you are planning to use has the CDL nipple to which the shift linkage attaches. If it doesn't, then you will have to source the CDL nipple. I think Rovers North sells it for around $70.
4) If you are not going to use the front propshaft from a DII in your conversion then you will need to change the front output flange on the transfer case. The size of the DII flange is too big for the DI front propshaft.
I am sure there are a few more issues that you would need to contend with when using a DII case in a DI or RRC. It can be done. I have sold a few of these conversions but before embarking on the job you have to arm yourself with an understanding of these issues and make sure you want to overcome them.
Certainly, picking up a DI case would make conversion in a RRC a lot easier but when has easy been the deciding factor....these are Rovers after all.
