Gareth: starting stalls were first used in the UK in July 1966, at Newmarket. It took a while for them to roll out to all courses and I don't know when Ireland began using them. So while you can compare some races from then, you can't compare any before then. You'd also need to be very sure that course work, such as dolling in or out of rails to smooth out sharp corners, filling-in of undulations, etc. hasn't taken place on the courses you're comparing and, for any NH results, the likelihood that since races even 10, 15 years ago, the water jumps haven't been removed and other obstacles relocated, as they have been changed at Cheltenham in the last couple of years.
Something else to take into account, and over which no-one has any control or can actually compute: Clerks of the Course, until 10 years ago, were relying on their walking sticks and their own 'feel' of their courses to determine the going. They now use the TurfTrax going stick which gives a standard reading based on penetration at dozens of specified places around every course, all of which are mapped out for the Clerks to follow, thus providing standardized testing over a period of several days. Years ago, Clerks didn't provide the amount of information to the public in the way they do now, and they certainly would not have gone round their entire course several times, testing in specific spots. They marched out a day or so before racing, pronged the ground here and there randomly, and decided on pretty much how it felt. Nowadays, they have specific places to probe three times in each place (giving an average for each designated area), meaning that some 180 probings might occur each of the days the Clerk examines the course, and then an average of the whole course, based on some parts being Soft and other being Good, for example, is arrived at by the stick. You can be damn sure that Clerks of the past didn't spend anything like the time now taken to probe the entire course not once, but from five days out to the morning of racing. What does this mean? It means that back in the 1970s and 80s, for example, you might've had any amount of Flat and NH Clerks who merely jabbed their sticks into two or three places on the straight, the turns, and the back straight and came back with 'Good', whereas if they'd examined a number of other spots on the course, they'd have come up with "Good, Soft in Places", which naturally has a bearing on, say, a 3m chase.
Another issue why horses "back then" had a lesser advantage over horses today is in the care of the tracks themselves. Today sees Clerks favour a mix of rye and other close-growing, sturdy grasses which can take a hammering. There is immediate divoting on the day and directly after. There is watering with boom irrigators or more sophisticated systems and there is far more emphasis on de-thatching, re-seeding, and making sure that tracks are grassed all over, without bald patches, which make the going uneven. Look back at some of the pictures from even the 70s, and there is plenty of thin, dry grass on summer racedays on the Flat (summer jumping not on the cards back then). So "Good" back then would've been closer to "Firm" today - not that any Clerk ever likes to call his going firm, as it frightens away too many trainers!
So, if horse A won in 1976 on turf, and you'd like to compare his performance on the same course today with horse B, you might find you have to figure in subtle changes to the lay-out of the track, the going surface as called is not assessed in the same way today (thus the calls would mean different things), and the actual grass surface is now different underfoot thanks to a different type of grass and a better watering programme, and, if you want to compare NH values, be sure that since the old horse's contest, a couple of jumps haven't been taken out since then - which obviously changes the race timing.