Fred, I had the very first K27-HF with the 2 3/4" inlet on the compressor. It was a nice little improvement over the K27-7200 I originally used on my car and had better top end.
When I decided to go with the Garrett GT35R, I also replaced the OEM heat exchangers with a nice set of aftermarket stainless steel headers, so the vast improvement over the old setup would have been a attributed to a combination of the headers + ball bearing turbo.
The Garrett GT unit will bolt right up to the headers, however, you will have to modify the inlet flange to your muffler and also run a smaller oil feed line (-4AN) since the Garrett doesn't need as much oiling as the KKK units.
The GT turbo also has a different oil drain flange pattern, so you'd need to have your oil drip tank modifed with a GT drain flange on it.
If you're tracking your car, I'd recommend running oil through the water passages on the GT turbo. For pure street, they are safe to run oil cooled only. Just leave the water ports open.
Whether you decide on a K27HF or GT BB unit, it'd be a vast improvement over the factory 3LDZ turbo/paper-weight
Mate the new turbo with a nice free flowing muffler, and you're good to go
'89 911 Turbo Cab
Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition