on this kind of situation, it's really just what you want. LS/VTEC is a good conversion for turbo applications. The only disvantage of the LS/VTEC is that you have to be as careful as you can, during high revs up to 8200 or more, you motor will tend to blow, due to because it's not to be built that way. (most LS/VTEC i see blow when reaching 8300) not to hate on the LS/VTEC proud owners, just commenting on the motor...if your thinking about LS/VTEC, build your block as best as you can, your head will need to be built to handle turbo applications too.....
As for the GSR, you dont have to do any hard conversion, just drop it in and wire up....or while the motor is not dropped in yet, just wait and tune the motor up, then drop it in....it's hecka of a less hassle...GSR are mostly great for running N/A tune up....you can get up into the 11 secs with the right modifications.....i have Toda/Spoon fully built with only 262 hp, and running 11. so it's up to you what you wanna run. BTW, i have been pulled by LS/VTECs turbo before too.....make your decision....