just another possibility
I've hawked this thing in a prior post... It just seems to have a lot of the concern taken care of for balancing a wider part like the crossmember. I could see easily modifying it too. The jigs are sweet and certainly better...but I personally don't have the skill set.
But... 'what if'... the side arms are extendable on this. they also have spin adjustable pads, but if you turned the arms upside down, you might be able to place bolts right in there as guide pins(?) It's $40, I'll be trying if necessary, but so far haven't needed to...yet.
If the arms are too short, one could add newly jigged arms, or matching steel arm (hopefully a common dimension) in place of these shorter ones and drill some pin holes at the ends. What's most nice is that the fulcrum is bolted to the jack lift pad...I just like the look of that.
http://www.harborfreight.com/floor-jack-cross-beam-60762.html
Let us know what you work out.
DD (CA)