If you are using a "period" battery box to house the modern battery, then the exact internal dimensions will vary according to the type of box and you will need to measure.
If you use an AGM type, then it doesn't care about it's orientation and I have some mounted on their end in my boxes.
Like Johnny, I have found that the AGM stuff lasts alot better than the small lead acid equivalents
Not connected with them, but I use tanya batteries off EBAY as they are well priced and arrive quickly:https://www.ebay.co.uk/usr/tayna-batter ... 7675.l2559
You will need to know the number, of the battery, unfortunately to order.