Some good points on machining after a few months of heat cycles
bracket for extra support is a given and what I have used for
cast iron recently is brazing using the Oxy successfully.
Brass is not cheap but cheaper than high Nickel based rods that
are usually avail for this kind of mods.

I would make the exhaust manifold as small capacity as possible
using a bigger than logical turbo to make sure it spools faster
for the street where Logs are enough below 6500rpm redline.
For racing where the upper rpm is needed equal length are king.

Just for comparison a G13B suzuki gti motor is using a T28BBRS
E85 22psi for 220kws, thats 1.3 litres but nothing below 3500rpm.