Is there another tub elsewhere in the house, and is there ANY chance that resale may be an issue? Most people do believe pretty strongly that having no tub at all is a very significant defect in a house (and not everyone wants to have to get right into major renovations when they move into a new place), but if there is another tub in the house removing the tub entirely is a viable option.

If the light from the window is important to you (it would be to me), then by all means keep the window. I know there are vinyl-framed, preassembled window-block units that are quite reasonably priced, that would be a lot more sensible to install than trying to block the empty window hole in with individual blocks. IIRC, the acrylic blocks don't transmit cold from outside as well as glass, so check into that.

There are MANY nice soaking tubs with integral tile flanges, which would allow you to tile the wall around the window and not fool around with tub decks (which is just asking for leaks). One with an integral apron would give the cleanest appearance IMO and be simplest to install. Attractive tile can be had, to put it bluntly, dirt cheap (like under a buck a square foot), and assuming you don't mind the grout care it does look so much better than a fiberglass surround. If you want to fill in that whole 6' space, check out the Kohler Mariposa K-1259 or 7236 (both of which have a separate apron which will match the tub exactly); I think those are probably the most reasonably priced of the 6' tubs that are appropriate for this installation. If they're a bit too rich for your blood and you want to downsize to an inexpensive, standard tub, you could have a carpenter frame in the ends to create a space for a standard 60x36 tub, keeping your window centered. (You can put sizable built-in shelves in niches at the end of the tub opposite the shower now, too.) Don't even think about blocking off the window with a conventional surround, since you can tile I think you should just get a nice tub with an integral tile flange and tile the walls.

If you remove the tub but you have any eye toward resale, keep the shower big. People are really digging the big showers these days. There are shower bases (aka receptors or pans) made in a 72x36 size - I found a really nifty one with a built-in seat from National Bath. (Don't freak about the list price. Not that many stores actually charge list price.)