This pretty much explains everything about a vertical grow including how to make one;
https://www.rollitup.org/hydroponics-aeroponics/149998-heaths-flooded-tube-vertical.html
I'm not sure you need to upgrade your light. The difference in what you have versus buying a 600W isn't going to be much. Like I said, you can put your bulbs closer to the plants because they won't be as intense and hot. You can only fit so much into a 3 X 4 space, and heat is going to be an issue. Since you can go tall, you may want more than one bulb,.. to light different areas. Where a 1000w is better is spreading more light around,.. but the benefits for just the top cola's is arguable because you have to have the light further away. The further from the bulb the less intense the light, and the circle of high intensity from a 600w is much larger than a 400W.
A 600w can light buds on the plant that are lower and on the sides. No bulb can put light on buds within the plant or under leaves in shadows. Since you can put a 400w closer, the top buds could receive virtually the same intensity of light, but it won't light up the side buds as well, or put as much light on plants not directly under the light.
If your'e going to do a vertical grow,.. you may just want to use two or three of the walls. Put reflective material on the others.
Heath's grow is 4 feet on the interior of the plants meaning his space has to be 5 X 5 or larger and he didn't quite get 3 lbs. My advice to you is forget the 3 lb target, unless you can expand your space, or set it up so the vertical grow goes 7 feet up the wall.