Overnight train from Budapest to Split in July

Reserve the seats first. You only have to make sure that you add the train to your pass and activate it before you actually get on the train. 

But since you mention a change in Zagreb, if the change is after 00:00 you will use 2 travel days.

Assuming you're looking at the 15:25 from Budapest, you can also change in Vrbovec, where the night train leaves at 23:40, so you'll only need one travel day and can sleep a bit longer.

Reservations for this night train can only be made at a ticket office in Croatia or by e-mail to Croatian railways, see:

https://www.seat61.com/interrail-and-eurail-reservations.htm#Croatia

Since sleepers are so cheap on this night train, don't hesitate to book one.

Alternatively, there's a direct night train Budapest-Split on 7/7 and on 9/7, departure 18:40, which can be booked on the MÁV website.

Be aware it takes a bit for the reservation by email, had to change my recent travel trip a bit because it sold out before i managed to get the reservation. Also no AC in the couchette cars (when i took the train 2 weeks ago temperature were still comfortable in the night after opening the window. (but that was only the case for the trip to split the way back was hotter but i did that trip with the EN.)

If you are short on travel days a normal ticket for the croatian night train in couchette is only arround 22€ (includinding the 7€ for the couchette reservation) 

The sleepers sell out pretty early on that route.