I'm assuming it's a B230 not a B23 in your 91. If you want to put a turbo engine in the 91, you'll have to swap plenty of parts whether you go with a B230FT or a B21FT. Several components are different between the LH2.2 systems for the turbo & N/A motors, among other things the ECU and injectors (which also require a resistor pack) are not (supposedly) interchangeable. However, the B21FT is a rather more involved job to install. You'll need the main fuel pump, wiring harness, both ECUs (the CIS & the Lambda), the ignition, and fuel pump relay & its wiring. I think, in your position, I'd go for the B21FT. You've got the donor car right there, you have all the parts you'll need and it's just a matter of labor on your part. And the B21FT is, in my opinion, probably the best motor Volvo made. I have never killed one. I do have an ever-growing collection of broken B230F/FT rods - they're still great motors, just not as bulletproof & tolerant of abuse as the B21FT. And K-jet, once any bugs are worked out, is fantastically reliable even if it doesn't deliver the same mileage as LH.
-Chris