I get this - it's a fun project with some nostalgia thrown in... but.
Getting lower gears on old bikes is one of the most common questions we get in here... and the Mechanics subforum.
The answer is it is almost always too expensive for what the owner wants to do... likely cost more than the new purchase price of the bike back in the Reagan administration.
Crank, axle and BB, shifters, front and rear derailleurs, freewheel... on top of the normal tune-up stuff like chains and cables and brake pads.
A 1984 Peugeot might have 27" wheels; there's another big expense.
If you're an experienced wrench with lots of resources you could probably do it anyway.
If this is your first restoration, you'll likely find it frustrating and unfulfilling.
I know you like it. There's another bike out there that'll make you happy.