Instead of putting a ratchet strap on the alternator bolt, which puts un-needed stress on the factory cast aluminum accessory bracket which is as rare as unicorn's teeth to find when you break it, I would suggest RJC's HD rear motor brace kit. Works great in my exp and the engine only moves as much as the rubber bumper can flex, which is basically none.
rjcracing.com
Downpipe hitting the control arm, and or the firewall to floor pan seam is very common. With regards to the firewall / floor pan seam, on any "new to me" Turbo Buick, I check to see if the previous owner / owners fixed this. If not, I break out the Dremel tool and 8# sledge. I notch that damn seam, hammer it down flat all around the downpipe and transmission bell housing area that I can get too, then shoot some primer and black paint over the area to keep it from rusting. Effectively killing two birds with one hammer. And said tools and paint come out again when the trans gets swapped/rebuilt, in order to completely hammer the seam flat. Your hands will thank you.
Hope this helps.
Patrick