You should be able to guesstimate your original , measure the length from the rod as it enters the master to the bulkhead , then the bit that is in the engine bay , then calc that x 5/8 (32mm) and you have the total volume of fluid moved (roughly)
Then the length of the new one x 1/2 (26mm) and if close , it’s possible.
I’ve seen the 1/2 inch master for £25 so not the end of the world but not 70mm , seems 57mm is common
Either weld tabs to the new one and drill at 70 mm or make steel tabs and bolt to the master using original holes and drill new holes at 70mm use something like 1/4” steel for strength so it won’t bend
Sure it will be a faff but you will be a pioneer and that’s always a faff !
the one I linked is 105mm long , so about 1/2 of that would be the piston 52x26 =1352
So your original you are looking at around 84 mm length so 84x42 =1344
But it’s guesstimate to see if it’s possible