8 1/4 Install Questions

I appreciate any guidance you can give. The 7 1/4 has been removed and the 8 1/4 installed with new u-bolts, thick washers, and barrel nuts. I have 2 questions (well, maybe 3 or 4). In trying to remove the shocks the top nuts are both well frozen. Can I use my 36” breaker bar or is there a danger of snapping them off? Is the mount welded to the floor pan or is there another nut I can access from in the trunk? This car (67 barracuda conv) has been sitting since the 70’s. Next question is I noticed a broken leaf, and the spring bushings look like Hell. Is it just as easy to drop the springs with the rear installed or should I remove the rear and work on them 1 at a time? Any tricks I should know about in replacing the springs? Like I said, any and all guidance is appreciated.