I've heard a lot of people say that the cowl panel is different between rain catchers and flat hoods but I'm not sure I believe it. If ya can find a hood, ya can find a cowl panel and they're usually pretty cheap. Would ya believe I scored a flat hood for $30 a couple months ago?
77 and 78 were the only years for turn sigs in the grille so ya need a lower valance with turn sigs from any other tin grille. There are minor differences but I want to say that's all about the screw holes along the bottom edge of the grille. Maybe a bit of a profile difference in the same area but not enough to be a show stopper.
Rad supports are year range specific. In this scenario, 79 and 80 are the only ones stamped and punched to accept the quad headlight setup. One cool thing is that dual and quad use the same support. Single buckets are spot welded on, the quad piece bolts on and yes you can hack up an earlier one and make it work but you'll only get 3 of the 4 mounting holes for the quad buckets.
The other thing you need is an adapter pigtail for the head lights. 79 / 80 used the same under hood wiring harness but different pigtails based on dual / quad head lights. When ya grab the rad support with buckets, cut the connector off the harness and splice it in to your older harness. Ya'll probably need to extend the turn sig wires as well.
One potential ugly in laying hands on the parts is the LRE guys. Quad setups are kinda hard to find and often get priced up because people sinking money it to a Red resto will pay up. Every once in a while you can stumble across a complete set up for cheap. The one below was on craigs for like $300 last summer. Ya can see the repairs if ya look close but for the coin, it would be a no brainer. Push comes to shove you can splice a couple rad supports together and I plan on doing that with a couple of my spares when get I get my welding skills together. Bottom edge from an earlier hacked and bent support that's mostly rust free. Top part from a 79 / 80 that wasted along the bottom.