big port mani wont fit small port head so scratch that. dont even worry about o'ringing the block either. just machine all th surfaces and then run a quality headgasket. wouldnt worry too much about the head either, just make sure its in good condition.
as for everything else.. go for it.
your motor will handle whatever boost you want to run.... its all in the tuning as to if it can hold together or not.
but that said. the T25g is shit and wont do much more then 14 psi without falling apart and even then thats right on the edge of the efficiency map.