Since no-one else seems to have suggested it yet I'd say kill off the shuttle. Built on wildly optimistic claims that should have been treated sceptically even at the time, massively overcomplicated and horrendously expensive. For all of their being a marvel of technological achievement NASA would have been better off sticking with rockets in my opinion.
The Shuttle is a hard one. Does the technology exist in the 70's to create a reasonable re-usable space vehicle? Then it also becomes what are you going to re-use?
In my opinion SSTO just doesn't pencil out when you do the math. You really need a leap in material or propulsion technology.
This leaves me with the next best option, TSTO. Certainly by the 90's the technology was their to build a reasonable TSTO vehicle that was fully re-usable. Was the technology their in the 70's to do this? Maybe, but then it becomes how much do you want to spend to get their? In order to really drive down costs and get here.
You need a full re-usable vehicle. However can you accept landing some place in-between?
The first possiblity, just a re-usable spacecraft. Maybe something like the DreamChaser. You have a rapidly re-usable spacecraft and no service module thrown away after each flight. That get's you some savings but you still have all the rocket cost.
The second possiblity, re-usable spacecraft with the 2nd stage engines on the spacecraft. So you are re-using the 2nd stage engines. Something like if you stacked shuttle on a Saturn-V. Remove the 3rd stage and place the Shuttle on the 2nd stage. You remove the engines from the 2nd stage so it is just a fuel tank so it just becomes a fuel tank for the Shuttle. The initial push off the pad is managed by the S-IC. However you are still throwing away the S-IC (F1 engines) and the 2nd stage tank with each launch. However your technological challenges are not that big and you are re-using hardware.
The third possiblity. The 1st stage becomes a re-usable booster with a crew. (Don't trust 1970's technology for a re-liable flyback for the booster to a runway). The 2nd stage is still just a tank with the engines on the spacecraft itself. You have now introduced the challenge of the flyback booster which is a hard technology leap, not impossible but hard. The 1st stage booster is 1,000,000+ lbs hypersonic vehicle that we can assume at separation will be hauling at around 50 miles up and mach 8-10. This will require test flight work to figure out how the vehicle will behave. You also have to decide for boost back to a landing site do you come back into the atmosphere and then have air-breathing engines igniting to get you back to the launch site? Or do you reserve some fuel for boostback and use the rocket engines to get you back? Not sure which is best. For me the boostback using the engines is intriguing because it removes the complication of air breathing engines and separate fuel tanks. At this point you are dumping the tank for each flight but you are re-using just about everything.
The fourth possibility. As above but the fuel for the 2nd stage of launch is moved into the spacecraft. At this point you are not throwing anything away but you have a lot of technology risk. You not only have to develop a 1,000,000+ lb hypersonic booster stage you have a orbital stage that is attempting to deal with the large volume of a LH2 tank (assuming you are using LH2 to get into orbit). You then have to bring back this volume into the atmosphere with all the weight of the rest of the equipment. Also to be successful the equipment has to be easily re-usable. Which for example the SSME's were not easily re-usable. This cares of lot of technology risk and high price tag for doing this on 1970s' tech.