The problem with 660s is that you will lose control of timing if you go with them.... yes they will give you the fuel you need, but if you use a standard piggyback like an SAFC, you will have to deal with a hell of a lot of timing advance.
Big front mount AND a better Y-pipe??? HUH?
I would stage the upgrades, get the ones you absolutely need before going up to bigger turbos, Get the turbos, then get the rest which you need for the power, but not necessarily for the bigger snails, this way you can work your way up to your goal with incremental increases, instead of waiting a while, then landing there all at once.
Things to get before the turbos:
- A REAL boost gauge
- Fuel pump (I am partial to the Supra pump myself)
- Hotwire for the fuel pump (not necessary til later, but its cheap and easy)
- Fuel controller and injectors (you need to install the controller first, or install them both together)
Things which are nice to have before the turbos:
- Exhaust upgrades (Down pipe, cat back, "high flow" cats)
- Electronic Boost Controller (partial to the DSBC)
Things you will NEED shortly after getting the turbos:
- Clutch (it may last a short while depending how you drive, but you will burn through it)
Things you can do after the turbos:
- Intercooler upgrades (it will give you the "icing on the cake" as it were)
Good luck.