Find a cab with greenbacks. I prefer the Basketweave Green Mix. Put a TS-808, a Fuzzface, or a Big Muff Pi in front of the amp. Make sure that your that your Master Volume is all the way up. The '72 didn't have a master volume. I don't know whether this is one of them with the tone stack in front of the gain stages like the the 1987XL or the JTM-45, but it doesn't sound like it is. On those, you dime the EQ. On this one, I set it at 4850.(i.e. Bass/Mid/Treble/Pres) to start dialing it in. Set the gain on both channels around 5. A Plexi on 4 will take your head off, so most people didn't play it cracked open much farther than that. Eventually, I wound up somewhere around 4724 with the TS-808 on 5 and 5. Use the TS-808 tone control as a gross EQ control when switching guitars and control your distortion level with that by using both the volume and the distortion knobs. You can also use the treble side gain as a treble control and normal side gain as a bass control, so you can mix the differing distortions. If it sounds muddy, turn down the bass level, because you are getting overtones of the bass frequencies. The thing to remember is that the tone stack on the Marshalls were pretty cross coupled.
Also, remember that there were no effects loops, so any effects had to go in front of the amp. In those days, delays were tape based or analog bucket brigade type. The tape delay can cut some of the top end off of the amp from replicative fading, so you can use that to make things smoother. That's how you head toward the classic 70's lead tone.
Tell me how it sounds to you. And, we can work on issues from there. The problem might be your pick ups are not hot enough. Fender laces were pretty low output. I don't have 90 Strat Plus anymore, but I was just checking out things with a '69 Strat Relic. That guitar went right there.
