Water meth setup would do nice for keeping it cool when you get into the high boost, but really if you are going to get into the expense of custom manifolds, get a new GT series turbo. With either a 20 or 22, a 20 would be a good place to start as it would be a good high pressure turbo for twins if you ever decide to go that route. :wink:
If you are determined to keep the t3 a .48 is about as big as you want, if you are going to keep it in normal rev range's, if you are planing on running this more like a gasser into the 6000 rpm+ where a .48 or bigger becomes valuable, then you should look at having the valve relief's machined deeper 1mm or so and get Dave to make you a bigger cam to go with it, as well as HD valve springs, and retainers. If you are going to build the whole engine put ARP or Raceware Mainstuds and rod bolts in it, and look at getting you rods at least shot peened and maybe polished.
What I am getting at with all of this is, unless you are willing to push the envlope .48 is as big as you would want, and may still be as big as you want in a street car, in which case the best way to get more boost and cooling is to go twins.
O-ring both the head and the block not just the head. Oil coolers and fuel coolers are highly recomended as well.