So you know, a on-center housing will change the position of the turbo when compared to a tangential (much more common) housing. They both bolt onto the same flange, but the compressor side will end up being about a 1" difference in position because of the shape of the turbine housing.
The downpipe is trickier. There is the most common "V-band" which SP turbos and Turbonetics use. Most Garret tangential housings use this discharge type.
On-Centers turbines use a circular 4 bolt discharge.
Greddy and HKS use their own square 4 bolt flange.
You can have the downpipe modified cheaply though. For instance if you take a Greddy downpipe, cut the square flange off the very end, and weld on a $20 v-band lip, you could run a Turbonetics or SP turbo on a Greddy T78 kit...minus the T78.
Note...the above discharge types are not ALWAYS true. You put a circular 4 bolt flange on a tangential housing, some housings have a "slip on" discharge that allows you to put your choice of discharge types. This is done rarely (seldom there is a reason to), but CAN BE.