The 250 is a low deck engine and you cannot "convert" it to a 292 as you can't get enough stroke out of the block. A 292 is a different block altogether.
Sounds like you have a lot of options you are thinking about . . . time to really think long and hard about what you want when you are done. If the engine runs now - work on other stuff till you have it figured out. 69's came with a long list of engine options including a couple of different small blocks and at least a 396 big block along with the L6 options. I had a 69 w/T400 stock rated at 275HP and it scooted pretty good and got about 11mpg with my foot in it all the time.
