Pretty sure that silver box behind the brake power booster (on the firewall ) is a starter relay/remote starter button..I don't have one on my '77 Pontiac T/A , but I've previously seen one on a '74 Pontiac G/P ..It allows you to crank the engine from under the hood 😉 Thumbs Up on the progress ! 😇❤
It's a horn relay. Sometimes people use it to grab power for other "always on" devices. On most GM's the starter relay is physically attached to the starter motor and is called a solenoid; I think it's called a solenoid because it also tells the starter to advance an arm with a gear that engages the flywheel. It is possible that you are thinking of the remote black, sometimes silver, solenoids that Ford and Mopar uses. People retrofit them to GM's because it solves heat soak issues associated with the starter.
The subframe is drooping because the frame bushings are completely shot. Take a close look at the holes on the subframe after removal; along with rust, over time the old bushings literally drill their way through the frame. The holes should be a little over an inch but if it's closer to 2.5 inches (the outside diameter of the bushing) then you need to weld in some repair washers; I'm not certain on those diameters but they be should close. You can buy the washers in a kit from the usual F-body suspects or you should be able to source them from a large truck parts supplier. It's a lot easier than it sounds if you can weld otherwise, make friends with a welder. Also, when you remove the subframe, lubricate the ever loving heck out of the caged nuts and DO NOT use an impact wrench or a cheater bar. Take your time moving the bolts back and forth with steady pressure; it will take quite a bit of time but it's better than having to cut up the floor pan to access the caged nuts. If the bolts already spin freely then the cages have been broken and you will need to slice open the floor. Also, be prepared to find some rot under the windshield. I had to remove the cowl panel on a few of my projects to repair the A pillars that we're held on with a little more than a prayer. Did I mention that this is a good time to take a few welding lessons? ;) Just an FYI, farming out labor for a car project is gonna absolutely kill any kind of budget to the point that it would make more sense to buy an already restored car. I'd recommend you buy a grinder with a few wire wheels and flap disks. It should only take 6ish hours to dress and prep the subframe for paint. You can buy quality, brush-on DTM paints if you're not setup for spray, look for chemical resistant paints; Rust Bullet is one recommendation off the top of my head and beware of the marketing wank that goes along with POR-15, not sure what you would have available in Europe. IMO, powder coating is not worthwhile; it's not any stronger than a regular paint but it does look nice. Try to avoid spray cans as they usually are not gas or oil resistant and are incredibly thin unless you're gonna do 5 or 6 coats which gets expensive in time and money. Sorry for the wall of text; I'm a huge 2nd gen f-body fan, especially the 70-73 'Birds. Get on a few message boards specific to F-bodies, they will save you a lot of time, money and anger.
Just wondering the round hole where the heater motor is won't water get in as not being closed off even with a mesh water will get into the area n cause rust
