The Ford Cargoes had exhaust brakes available as an option, if my memory serves me right, so I'd expect a used one would be available from a lorry wreckers to suit your engine. You would need to get all the exhaust system, wiring and air piping as well - there are usually two solenoids to cut out the exhaust brake, on the clutch and throttle pedals. All exhaust brakes have some sort of on/off switch which keeps the valve open but is over ridden by the clutch and throttle peal switches. An air supply is the usual method of controlling the valve, with electrics opening and closing the air actuators. Any competent truck mechanic could fit one to your vehicle, as long as there is space to fit it - they bolt up to the exhaust manifold where the exhaust pipes start. Bear in mind that fitting this to a non tilt cab will be harder than fitting to a truck as all work has to be done from the underside or through the cab floor. I'd say about 20 hours of labour to fit, maybe another 2 hours to iron out problems.
Buying an aftermarket exhaust brake and expecting it to work is going to be a bit hit and miss. Exhaust brakes are designed for a particular engine - ie the volume of exhaust gas flowing through the partially closed valve - how the engine performs with the valve open, etc etc. Just by putting in a restriction in the exhaust system can have major performance problems with loss of power and increased fuel consumption. An eBay exhaust brake may not be a bargain.
Exhaust brakes are great, they certainly drop the amount of service brake usage, especially on long downhills. Contrary to popular myth they are excellent to use on loose or slippery surfaces (4x4 fire trucks in Australia have them fitted since early 1980s) - they are not ideal on ice or where there is no traction - but brakes are useless anyway.
The Jcobs Brake is a completely different method of engine braking by controlling the opening of engine exhaust valves.
