The main journals on the 400 crank will need to be turned down from 2.650 to 2.450 to fit the 350 main saddles. You'll want to have the crank rebalanced as well. It will also require the use of the 400 balancer and flexplate.
Due to the factory's poor balancing jobs on these cranks it's popular to go with a aftermarket crank from Scat which you can get in stronger cast steel as well as ready to drop in your 350 block. Plus with scat cranks you can chose to go internal or external balance as well as 1 or 2 piece rear main crank seal. With the cash you'll have to invest on the stock crank to make it usable, you're better off going with the aftermarket shaft.