Crazy how much hate there is for big wheels!
20's should fit. Whats the tire height on those from factory? 30"? There should be plenty of meat in the tires. A 20 in a 30" wheel is like an 18 in a 27" wheel. its about proportion.
I don't know if the 68 and 73 have any similarities but I test fitted a set of 18"s on my 68 to help work out what size and offset I needed and I found the following fit with heaps more room to spare toward the guard (so even less offset): 18" x 8.5" +25mm, 18" x 9.5" +23mm
Bigger rim usually means you can get a wider wheel/tyre due to the lack of 'ballooning' from the side wall.
As mentioned, it will be the width and offset/back spacing that you will have to be careful of. Also if you scrub, you'll most likely be rashing your rim lips, so when you measure up chuck a heavy guy or weight on that car side to help simulate some roll etc. Try and avoid it as much as possible.
If your looking to get a flush or close to flush fit, consider rolling the lips around the wheel well opening back so there is no protruding metal.
Also some considerations besides aesthetics:
Less side wall means: Less side to side movement in the wheels side wall for better handing around corners. Less give in the tyre to take bumps, so generally a harsher ride, which can wear the car quicker.
Bigger wheel usually means heavier! and rotational mass is more damaging to acceleration then non rotational mass. 10 to 1 if I remember right.
Tyre cost. Usually, sh!t loads more!
Past that, I saw a late 60's ch3vy the other day that had massive wheels on it and I think it pulled it off. Looked like a scale hot wheels car! Cool if you going for that look, but I'd wanna make the rest of the car suit, or it could look off putting / out of place.
HTH