You can trim it back as much as you'd like. You won't kill it by pruning. I don't know if the rose is on its own roots or if it's grafted but cutting it back might even give it a new lease on life.
If the rose only flowers once a year then wait until after it has finished before pruning it. If it flowers off and on all summer then I'd wait until either the fall or, even better, the spring to cut it back.

Note tha a 70 year old bush may only be producing new canes from farther up on the trunks. I would look for where on the heavy, woody stems the new green canes have been coming out, and cut it about a foot above there for the main pruning. With luck, that will stimulate new canes farther down, and you can reduce it further next year.