I have followed this thread with some interest as I have a couple of Bills and a couple of Toby the Tram chassis ripe for conversion. I have to say I am very impressed by the running qualities.
I have a considerable number of chassis from American manufacturers as well and apart from the Bachmann Porter chassis (which is truly excellent) is the best of the lot. I am currently building a model of Sir Arthur Heywoods 'Katie' and can report no problems with fitting a body to the chassis.
The only minor problem is that the motor protrudes too far forward, so I loose the daylight under the boiler. The footplate sits perfectly on the top of the chassis which is uncannily at the right height. 'Katie' is longer than the chassis so I loose the ability to use the Hornby couplers. Which I might live to regret as I am still looking for a reliable auto coupling system.