Sparky"
Yes, parallels will give you 400 amp. Now, depending on the load to be connected (quan. of 277 lighting, if ballastsed) will determine if the neutral is a current carrying conductor.
You neglected to state the insulation on the existing wiring & pipe sizes.
310.15 (4)(c) 2002 NEC is the article you need for the explanation.
Although not NEC required, & dependent on the conduit size, you may want to consider 4/0 THHN Cu., and a 'full' 42 circ panel.
(Not that much extra cost for the additional space) Remember the cb's are pricey also.
Hope this helps,
John
