In my experience tomatoes are not strictly photo-period dependent when it comes to the shift to flowering though it does contribute. There are a number of factors that initiate fruiting on most vines. Photo periods do mean something to them and I would go ahead and reduce your light cycle to a 14-12 hour day. On top of that, temperature difference between night and day is important. A 10 degree 'snap' between high noon and the coldest part of the night is usually sufficient. We peak around 75 degrees during the day and 58 degrees at night in our indoor rig and have faced no problems. Also, nutrient content makes a difference. Tomatoes need huge quantities of potassium (delivered as potash, K2O, in most fertilizers) to develop large healthy fruits. This is especially important since you're working with the meaty beefsteak strain. They also need elevated phosphates for the flowers. I recommend Bloom Burst at 1/2 the recommended concentration for phosphates and wood ash (or ideally corn cob ash) for potassium. On top of all this, there is a natural foliar spray that you can use to initiate flowering in tomatoes (and indeed many other flowering vegatables). It is Bonide's Tomato Spray, it contains the natural hormone cytokinin which is derived http://forums.gardenweb.com/forums/load/tomato/msg1222213220075.html?28from ascophylum nodosum seaweed only tremendously concentrated. I have had great success with this product not only in the initiation of fruiting but also in creating denser flower clusters and greater yields overall (so long as your nutrient and water regiment can keep up with the accelerated growth rates). This is the extent of my knowledge concerning your problem and I think it should serve you well. I am currently documenting my own indoor tomato garden over in the tomato forums, it may be of interest and/or helpful to you. I have included the link below. Good luck with those massive beefsteak vines, I would love to see some pictures!

Here is a link that might be useful: innovative indoor tomato garden

woops, got a little over-zealous with my ctrl+v command there. Huzzah for old-school forums that have no edit function, that'll teach me!

hautions - great advice! Thanks so much. I'll try positing pictures in a few days. I just can't believe how quickly these grew with hydroponics and artificial light!