From reports of others, the main thing that is looked for in the visual inspection is the air pump and air rail.
But some have gotten through smog without even that.
Regarding using '76 or '78 smog components, let me share with you what I have and then I'll pontificate
I have a '77 Federal emissions package, but I'm running a '78 Federal carb with a '79 distributor. My FJ40 *seems* to run fine, *seems* to have good power, *seems* to burn clean, etc. It passed the tailpipe and visual in 2005 and I got collectors plates so I haven't had to do anything more. In 2005, there were vacuum ports that were plugged (unused on my '78 carb and '79 dizzy) and went unnoticed during the inspection.
A note on distributors (copied from MUD):
•1975 & early 1976: small distributor, completely redesigned, very good quality points distributor. Vacuum retard.
•Late 1976 & 1977: Same, but w/ vac advance
•1978: redesigned w/ Med. size screwdown, waterproof cap, electronic ignition. Vacuum advance & retard.
•1979 - 1980: dual diaphragm advance. One big advance stage for normal operation, small second stage for extra advance at hi-altitude
A note on carburetors:
Different year carbs have different ports for vacuum control (somewhat dependent on distributor needs, whether vacuum retard or vacuum advance), the actual port drilling locations may change depending on what vacuum signal is needed. Also, items like the power valve have different controls (external or internal vacuum source) depending on year. Starting in 1978 a HIC vacuum port was added as well as a bowl vent. On mine those are plugged.
Having said all of that, obviously you can get by with mix-matching parts (like mine has-BTW I inherited it like this). But I can only believe that unless an informed decision is made about which parts to use then some goofy things could
There is also a consideration with the computers and wiring harness connections. The VSVs and computers do look a little different from '76-'78, so you would have to figure out how to accommodate those differences.
And then finally there is the little item where your hood and build date says that you have a 1977 emissions system, so if you change to another year then that could confuse someone down the road. '76 would technically be illegal, but '78 would possibly just confuse someone. And then to get a total '78 system you have to figure out how to do the HIC vent and bowl vent connections. It just kinda opens a can of worms.
You should probably try going to a mom and pop emissions place first (if they still exist), and see if you can piece the system together over the next year (your emissions will be annual with a '77).