Oh gosh – that’s a tough call — I don’t think there is an absolute right or wrong way to go here — the different options you have have their own pluses and minuses —
The tests in the 40’s are very similar to those in the 70’s — the test does not change much over time, and the underlying principles and issues that define right/wrong always stay consistent — that is, they don’t change what they are testing — having said that, the test does change a bit in certain other ways, in part simply because it is written by people, and these people change over time, and so the more recent tests will be slightly more representative of what you will see on test day than will the tests in the 40’s. That’s why the most recent tests are so valuable, and that’s why, ideally, you want to take them as the pt’s closest to the day. But again, especially because you do also want to be preparing yourself to face the less-expected, I don’t think it’s much of a concern if you have to substitute some older pt’s for more recent ones.
I think the determining point is how likely it is that you may retake — if there is a significant likelihood of that, what I would suggest is that you take three more practice exams — two in the 70’s and one in the 40’s (by the way, if you haven’t already, please take a look at the tips I posted for taking your final pt’s — http://thelsattrainer.academy/forums/topic/fourteen-quick-tips-for-your-final-lsat-practice-exams/). I don’t suggest taking too many more than 3 more pt’s — but obviously different study methods work better for different students and I’m sure you know better than I how to put yourself in the best position possible —
HTH — MK