how much work if its even possible without costing lots of money to swap a newer skyline engine in a 1990+ 300z?

You have a great deal of research ahead of you. I'll start of with the ubiquitous, "anything is possible with enough time and money." But no, for all practical purposes, an RB series straight six will not fit in a '90 300ZX. The engine is likely going to be too long for the engine bay, as the 300ZX engine bay is already cramped with just the V6 in it. Anyway, while the VG30dett is not as stout as the RB series, it is quite capable. So why bother with such a swap?

-Steve

you'd probably be better off spending the money on upgrading the vg, but it would be cool to have a skyline motor in a Z

dude your looking at a lot of money, first you have to find an RB at a descent price than you have to have your car cut for it, and new engine mounts make than all the rest of the shit, it would just be cheaper to swap your VG30D to the VG30DETT.....