I say disregard RarlyL8's (Brian) comments,(remember he's OLD & Crotchety!)
I'm working on a similar project myself and have recently decided to go to two turbo chargers vs one.
One is the "cool" factor, the other is that the Garret t3 turbos are quite plentiful (at least in the US) and can be had in many different configurations.The fact that they're plentiful means that they are also less expensive. For the price of two new T3's you could get a new k27. I've decided to do this even after purchasing a used k27!
As for the suitability of using those exhaust manifolds well...IMHO the primary tubes are too long, unless you're running at 6500rpms on an oval track, otherwise spool time is gonna be a pain.
(see the 935 above) The primary tubes should be kept as short as possible and YES I KNOW what Indy car turbo manifolds look like!.....different animal entirely!.
It looks as though you probably got those for free or real cheap anyhow...... what the heck.... weld a suitable flange on them and try them out, if the car is a pig (takes forevever to spool up) you'll know why.
Of course I'm ignoring the proper size,trim etc...of the turbos.
All of those specs will affect spool up time so..... consult the appropriate proffesionals.
I vote TWO.....