I have a 1973 Overlander which is 30 amp. You should have on your trailer just above the bumper on the left side a tag which tells you how many amps your trailer used. Stay within that amperage. Do not go higher.
I bought my 1973 last fall and bought a new converter after reading many threads about the old airstream univolt. I was patient and took my time researching the internet. I bought my univolt new. It was a 3230 30 amp parrallax which was discontinued. I got a great deal on it. It was under $100 dollars. I used the fuse panel from the old univolt and had it connected to the new parrallax. It works great!!!! I also had an electrician go over my electrical and had him put in a 50 amp 220 Volt electical panel seperate from my 30 amp box. If I ever decide to go camping at my property I may need that extra electricity. the 50 amp is not hooked to the RV electical system. I have a 30 amp plug for the Trailer and a 50 amp plug for anything I might need. Two seperate breaker boxes.
Check out the thread and you can see the converter I installed.