[Bryn Tirion]

Forgive me dear Mod, but I think you are not entirely accurate. I have made something of a study of this over the last 15 years, and have (I believe) every Chrysler, Dodge and Plymouth, including Canadian editions, of the workshop manuals for all car (not truck or motor home or combine harvester though) variants that took the specific wide body poly engines featured in the 407 to 410 models. At the bottom I attach a page from the excellent Bristol 408 parts book, which reproduces Plymouth manual pages in its engine and transmission sections, not surprisingly. The centre-outlet exhaust manifold pictured there was from the 1960 Plymouth (Canadian market) 313 c.i. engine. No changes. Bristol used this specific engine in the 407 and 408 cars, then an identical (I believe) 318 poly for the 409, but by the 410 had to use a different engine. This may well be what Bob has in his 410, and I have often speculated that this must be the Dodge Dart variant. This did have an exhaust manifold with the outlet at the rear (in the Dart.... nothing to do with Bristol!) which is why I'd like to see a picture of Bob's, but one must be cautious because the later LA engine also had a rear outlet, and that is an entirely different engine, although it would have been a very easy retrofit into a Bristol.
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On a related topic, I believe that all models from 407 to 410 used the Carter AFB 3131S carburettor, and I would be very interested to hear if there are other apparently original carburettors in these cars. In North America, the 3131S was only fitted to one series of cars, namely the 1960 to 1962 Dodge Dart with US built 318 V8. It has been claimed many times in the past that the Bristol engine had a Plymouth Power Pak option (note the quirky spelling), but this appears to be not strictly true, since that option (which for 1960 was called Super Pak) came with a series of other AFB carburettors, closely related, but with quite different jetting. If you look at the performance specs given for the 313 by Bristol, they are effectively identical to a stock 1960 Plymouth Belvedere sedan with the AFB carburettor option, although a quite different carburettor was used. Frankly I think Bristol just copied all the numbers out of the Plymouth manual.

At the bottom I have added pictures of the 1960 Plymouth Torqueflite buttons: note that they are round and without a Park button. These buttons changed completely every model year, and were unique to the particular range of cars they occurred in. If you own a 407 or 408 Mk.1 you will find that picture very familiar. They are specifically Plymouth and one year, 1960. I have also attached an extract from a 1960 Plymouth brochure discussing some of the engine options. As you will see, there is a Super-Pak option which closely matches 407 and 408 performance figures. And these were family sedans..... no nod, nod, wink wink, Bristol did some clever things to make those American engines perform better, as Mr. Crook was telling the motoring correspondents back in 1963 or so. Just standard American engineering.