250watt mh veg 400watt hps flower allowed?

I own two lamps from two previous projects both are and have mh/hps conversion bulbs. Id like to run them both so that I can harvest every two months. I also own two mylar grow boxes 1 approx 5x3x3 and the other 7x4x3. I used the 250 in the first box - they outgrew it so I bought the second box to finish cycle. This time my plan is to run the 250 watt in the smaller box for veg and the 400 watt in the larger box to flower. Then I can use my budget on multiple ventilation fans which can be attached to the growboxes. My question is, assuming the temp and humidity is stable, will the jump in wattage during flowering affect the plants positively or negatively? Might the larger box be too small for a 400watt lamp?

I think you will be OK. Temperature should be more of a concern for you. Extra wattage means extra heat.

If you've got temp stable as you stated, the extra wattage won't be an issue. Just keep the 400 up a bit higher than usual until you know the plants are taking it ok, then slowly lower as much as you can.

I am currently running a 400w HPS on my third day of flowering in a 4x2' tent, so there's no issue there either.

In fact, I went from a 135w LED with six 23w CFLs directly to a 400w MH in my veg stage and the plants loved it.

-spek