Yes, you can jack on that "Frame" rail.
It is a little stronger closer to the front as a fairly "armored" flat section goes from that thing to the front of the rocker panel area, under the cowl.
Spread out the load by placing a chunk of 2 X 4 or something similar on top of the jack pedestal to avoid punching through.
Actually, the frame looks pretty good in its current condition. Fill the confined space with an oil blast before winter, most leaks out, but it gets to where it is needed.
I have seen people fill that void with closed cell foam insulation (expandable) after the oiling, to seal it off from the salt and water in your climate.
