Inline 6 Turbo in a 1967 Nova. What turbo should I go with?

Hey I've got a build I'm working on. It's a mild one, so here are the limiting factors I must work with:
-194 Inline 6 bored .060. Iskendarian Turbo Cycle Cam 208 194 duration, 485 455 lift, 114 LSA. Catalog says it's good for up to 7psi of boost.
-The bottomend of engine is rebuilt but with stock components. 8.5:1 EGGE pistons(cast pistons)
-The head is ported and has new stainless exhaust valves 21-4N? or 21-2N? to handle a turbo.

The question is....what size turbo should I run? I've been told I'm probably limited to about 5 psi of boost considering I have a stock bottom end. Cast pistons being the limiting factor. Now I have heard that you can run a bigger turbo and get better fuel mileage because it wont be in boost all the time. So what would be good a T3 or T4 for my setup? And what would make more power with the same amount of boost(Bigger is better? sorry I'm not too familiar with turbos) And what part number would you recommend. I'm trying to stay within a budget so something affordable. Thanks!

most likely you are diving into uncharted waters .. you would more likely find help on an import site even though you have an american car .. lots of the import folks turbocharge the 6`s and 4`s .. i`m on a mustang forum and people are constantly wanting to swap a 6 for a V8 but i suggest to most of them to put a big 300 6 in out of a truck and put a turbo on it ..there is a toyota celica with the turbo 6 banger in huntsville making over 1100 horses ... we may not have the expertise you need ...so we may learn with you on this one ....