Yep, Lundahl can supply 'grid' chokes, as can Magnequest, SAC Thailand (SILK brand), and many other transformer suppliers.

As for driving 300Bs in parallel - quite a task; without doing the calcs, I think it would be pretty borderline... I think you would have some high-frequency roll-off at the operating points suggested, tho don't take my work for it. You could run a parallel driver stage, tho there are probably solutions that are easier to implement and less costly. I wonder what an E180F would sound like with a little more current (and lower plate resistance) - would stand a better chance of driving the parallel output...

Hope this helps in some small way.