Some interesting points raised here. After much research I too came to the conclusion that a newer transmission is the best way to keep the original character of the car but to significantly improve fuel economy.
For 409 and 410 the obvious upgrade is to a later Mopar 'box with overdrive and lockup. For a few glorious years there were hydraulically controlled models that can be automatically controlled with a couple of pressure switches and a vacuum switch.
Dick Peacock converted his 410, changing to an A518 / 46RH 'box. This was a straight development of the 727 Torqueflite and as far as I know mounts straight on the back of the Poly engine. In standard form the 410 would do between 18 and 20MPG on a run. Dick told me that he was getting closer to 30MPG on long continental runs. I hope the current owner of the car will chip in with some accurate up to date figures.
I discussed the subject with the gents from Classic Bristol Car Parts and they told me that for 409 / 410 the factory preferred the A500 / 42RH 'box from a 1992 to 1996 Dodge Durango. This is a development of the 904 transmission which is less robust than the above unit but is good for the output of the 410 in standard trim and can be rebuilt with more clutch plates to beef it up. The 42RH has a smaller torque converter but still bolts up directly to the Poly in the 409 and 410. The original starter motor bolts straight up as the bellhousing is different. The flex plate and dust shield from the 42RH will be needed as they different to the 727. 407 and early 408 have different crankshaft and will not directly bolt to the 42RH.
Fitting an overdrive will probably not involve modification to the tunnel but will require modification of the chassis crossmember along with shortening the prop shaft by around 3 inches. As the engines have good torque the factory back axle ratio is likely to still be suitable. Non overdrive units used 5/16" cooler pipes whilst overdrive units use 3/8" It may also be necessary to replace or augment radiator transmission cooler.
46RH transmissions can still be found in the US for reasonable money. 42RH are very hard to source but not impossible. STS imports will get a transmission from Florida to Essex for about £200 which is probably a lot less than the cost of crating and transportation to Florida from elsewhere in the States.
If trying to track down a suitable transmission it may help to know that A518 is the series with subsets 46RH (hydraulic operation) and 46RE (electronic operation) Similarly A500 has subsets among others of 40RH, 42RH, 42RE and 44RE.
Back to the topic - inlet manifolds more suitable for fuel injection adapters were made for the Poly 318 - E.g Weiand 7503 - but are very hard to find and quite pricey now.
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