It would be worth buying a small Bachmann wagon or coach and trying with a box. Once you've cracked it you'll be fine - but get a gauge - preferably the newer dual ended one plastic one which sets both coupler and mounting height. No affiliation to this supplier - it just came up first on a search.
https://www.hobbies.co.uk/kadee-ho-scale-insulated-coupler-height-gauge?utm_source=google&utm_medium=cpc&utm_campaignid=6727975578&utm_campaign=Shopping-Research_Campaign&utm_term=&adid=388174523725&addisttype=gpla&matchtype=
I've moved over to boxes for 80-90% of my stock as I need operational reliability..