What does your battery clamp(s) look like? On my 1980 it is a square frame that sits on top of the battery, the Halfords 038 fits perfectly and has proved to be a very good battery.
Edit: You answered the clamp question while I was typing it! For reference this is what mine looks like with the "frame", I don't know if your later model is different.
![[Linked Image]](https://www.tm-img.com/images/2020/06/22/038-battery.th.jpg)
As you have rightly deduced there is room for wider and longer batteries, it is height that is the main consideration. The 038 just fits but an 057 (I found out the hard way) is millimetres taller and will short against the bonnet.