The 105 grain Speers are one of my favorite bullets for my 243s. I currently use a load of 31.0 gr of IMR's 4064 in my BLR and it gives very good accuracy. This is not a hot load at all but rather a mild load that I use for fall turkey hunting. My theory is that a heavy bullet traveling at a modest velocity will not expand much at all, giving me FMJ bullet-like performance in a state that does not allow the use of FMJ bullets . I also get very good accuracy using 37.0 gr of Reloader 19 behind the same bullet. The long length of this bullet allows one to load it closes to the lands than lighter, shorter bullets, which I have found beneficial in my loadings.
I haven't tried IMR's 4350 behind it yet, but I bet it will work very well too, as 4350 is an excellent powder choice for many 243 bullet weights. I have had excellent results with it and 87 grain bullets.