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are there any 4 speed automatic transmissions that will bolt up to a 351m/400? and is there a way to convert a 351m/400 from a 2 bolt main to a 4 bolt main?
Any transmission that will bolt up to a 460 will bolt up to a 351M/400. For specific models you might want to check in the Transmission forum.
There are no factory or aftermarket 4 bolt main caps made for the 351M/400 that I'm aware of like there are for the 351C. Could you hire a machinist to make some and modify the block? Probably.
They aren't advertised but I was told you could special order 351M/400 4 bolt caps from Pro Gram for slightly more than the 351C caps. http://pro-gram.com/catalog/catalog.cgi
The caps are already pretty stout, you won't gain much with 4 bolt caps, Chevy motors are a different story.