We are talking std 4mt tank
The main problem is the height of coupling under the coal storage. For determining the height I used the kadee gauge, measure the difference with a vernier and then use this to produce the correct height.
I screwed a no 19 to the outside of the NEM box which if I recall is too high for the Kadee height gauge. I have a selection of small self tapping screws, in this instance I used a brass C/S wood screw. I have many thousands of self tappers in stock for fitting kadee's so if you want to buy and I know there difficult to obtain Mainly Trains are selling their 1/8" self tapers for £1.10 / 20 and £2.95 P&P I'll do 200 for £5.50 P&P @ £2.95 if anyone wants to order.