The TH400 is the toughest transmission you can buy dollar for dollar.
I too wrestled with whether to go with a regular 3-speed auto behind my 421 or to step up to an overdrive auto. After weighing the pros and cons of both I decided to just use a tall rear end gear and use the fresh TH400 that I already had sitting in my garage.
The added cost and installation hassles just weren't worth it to me, not to mention the extra couple grand or so that any overdrive auto would have added to my hobby car budget that's already starting to break the bank as I get closer to retirement.
I think a little while after your new big-inch engine has been on the road you'll find that those 3.55s are only good for scooting around town, out on the open road the excessive RPMs will soon drive you to install a 3.07 gear set in your 12-bolt.
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1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/3.08
2002 WS6 Convertible LS1/4L60E/3.23
1966 VW Type 2 Single Cab Pickup 140hp 2276
1956 VW Type 2 Panel Van 36hp 1200
1957 VW Type 1 Beetle 180hp 2276
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