You don’t say if your locomotive is of Standard Gauge outline, but do say it is freelance.

I take it Noel’s suggested height is correct for any standard-gauge 3.5″ buffer-beam and its fittings; but you can also find it on any suitable, published plant set or by measuring a complete example.

If you deviate from that it will be wrong and look wrong especially if you ever display the locomotive with scale rolling-stock or in the company of other 3.5″g locos. It’d give the rivet-counters – or Inspector Meticulous as LBSC used to call ’em – kittens!

Narrow-gauge outline locomotives, of any propulsive form, are another matter. I don’t know if there was much standardising with them.

That’s the loco end.

The truck end will be whatever is used in your club and may not be the same height, necessitating some sort of stepped coupling. The simplest, and as used in my own club, is a cranked, flat bar drilled at each end for the resective pin; but you will still need examine your local practice. You may find the rolling-stock already has multi-gauge couplings.

Those clevises, by the way, are nothing “special”. You can find them in many shops selling fasteners.