location is ALL about turbo size and the auxiliaries around it, and where you want to put an EWG if you go that way. generall there is a trade off in angle of turbo vs not hitting stuff just given the RHD nature of the cars here and size of housings on the turbo and screwing it round far enough to clear the intake runner and not smashing into the brake booster / clutchr res. This you will work out quite quickly putting a large snail in the hole to check.
DO NOT BOTHER twisting a VF turbo. totally pointless. gives you wank factor but thats it im sorry. twisting anything gt30 sized or a Holset or equiv and twisting is mandatory. I have seen people run a gt28xx in stock location with some sucess. they are quite fiddly to set up though.
With respect to the angle it alldepends on the turbo core and bearing type, obviously you can center them in the rotational axis as the housings generally rotate around the CHRA - depending obviously on the choice of wastegate design, interal or external. To know what the manufactorers permit it will vary from manu to manu.. look up on the net is my advise, but its bother all in the real word.
For garretts its something like 14-16 degs back to front off level, which in practice is bother all, and im yet to see a twisted garrett (on a subaru) sit within those tolerances .. unless the down pipe is run over the gearbox and down the passenger side firewall space. but thats very complicated.
On subarus in case you havent worked it out by now - swapping for a larger than factory turbo is a REAL bastard. from experience at least a 3 bolt flange on the uppipe / headers is a really good idea, and the uppipe requires a good amount of strengthening to hold a heavier turbo too.
Also from my personal XP the v band flanges on EWG\'s and Downpipes are generally the way to go for ease of fitmint and removal.
And ballsrburing has good practical advice above as well. 3" DP is where its all from anything above 250wheel KW.. OH something he hasnt done which is really recommened is putting a LARGE 3" flexi in the DP - allows for a good amout of movement with engine movement and heat expansion. With respect to uppipe diameter its best you talk to engine tuners / builders about your hopes and dreams, there is quite a few schools of thought on this. Personally i have reaonably large Uppipe, others swear much smaller is better. All i know is my motor has very good VE and it doesnt take a long time to spool quite a big turbo. YMMV.