As a
rough guide ..
Google what the 5.0 & 4.6
Mustang guys use..
For twins and rear / low mount T3 sized turbo with .62 turbine housing with ball bearing housing so there's no problems with oil supply or more the point oil return as ball bearing generally have built in oil restrictors [around 1mm]. The combined power from these turbo's would have to be 700 or so.. BUT you won't ever get that high in power. It's a theoretical max power level...
If you want to run a single then the GT3540 would spool nice with 1.06 housing or larger turbine.. They also come with internal wastes to keep things compact.. I would port the exit on it's housing to get good boost control..
External gates have far better control esp with big single turbo's ..
There are many others such as the T70 Master Power and Borg warner S300 /S400 series. They are oil cooled and shaft bearing and require much more oil to bearings. Which can be an
issue if using an electric oil pump to
scavenge oil back to engine if turbo's are
under sump...
If David doesn't mind ??
Here's a site that pretty much explains everything...
Search / surf through, most your queries
will be answered...
http://www.theturboforums.com/knowledge.php
Generally if using an external gate a size around 35 to 46mm would flow well enough..
As for blow off valve {BOV] ?? With in reason any size works.. Its there to prevent turbo spinning backwards when throttle is closed quickly and to keep turbo spinning while changing gears.. With out a BOV the turbine shaft can break or major damage to compressor wheel...