A few things to consider:
You need 2 types of clearance to go onto unimproved roads. The first is clearance under the axle. If you have a torsion axle that is shot, you should replace the axle. Yours looks like a Torsion. I would guess that any 1974 torsion is shot unless it was stored on blocks with wheels off. This will return your underaxle clearnace to normal. You are also going to need the travel of a new axle to take up the bumps you will encounter. Traveling offroad with shot torsion axles will send everything in your trailer onto the floor. If you have a straight axle, then you will need the right tires
to get the clearance. Your under axle clearance doesn't need to be any greater than your tow vehicle. If your tow vehicle is a Honda Accord you'll be dragging the muffler off long before the trailer axle drags! Measure the lowest point down the middle of the vehicle and try to match that on the trailer if you can.
The second type of clearance that you need is when crossing washouts or uneven terrain. This is where the front or rear of the trailer drags. This type of clearance can be improved by blocking the axle. I wouldn't go too crazy on this though! If you are really going to be crossing deep gullies, then you will need quite a bit, but otherwise, lower is better for towing. Remember that the talller it is, the more tree branches you will hit too! Most smaller 2 wd vehicles can't drag a trailer over very much anyway, so excess clearance is of no use.
One last comment. 4 lug axles generally are not designed for the added stresses of 14" tires
and wheels. If you go that route somehow, you should keep good watch on the bearings and hubs. Make sure they get regular lubrication and inspection for looseness. If you invest in a new torsion axle, consider a higher weight
rating with 5 lug hubs. It will be stronger and hold up better to offroad driving.