i was leaning towards a t3 as well. if i were you, since you have those turbos, do a twin t3 setup for you small block. it will make more power than the roots will, and if set up right, lag will be minimal, and the 400 should have plenty of low-end torque to make up for it.
as far as a box, all you really need is a carb hat. you can easily rebuilt a holley for blow through. you just need to change the floats, remove and seal up the choke, and bore out the accelerator pumps. there are a few more things to do, but i have a very good step by step write-up on what to do if you are interested.
want to sell a t3?
