I want to use a crossbolted 4.0 block but only have small journal crankshaft.

[62Healeyhacker]

Is there a way to adapt the larger, 2.5 inch, mains in the block to the crankshaft with the Buick 215" 2.3 inch journals?

[MGBV8]

Over the years, that is one trick that I have never come across.  It will be faster & cheaper to find a 4.0 crank or a 3.9 block.

[turbodave]

Find a block from a 1995 discovery - even better if you can find one with a build date in the second half of 1995, and it'll have the tall caps in the block. It won't be cross-bolted, but thats an easy machining operation for an experienced shop. You WILL need to line-bore afterwards, but again, no real difficulty for the right shop.

[ag1234]

More direct is better. Use 4L. crank=simpler,stronger and easier to balance.  Good Luck, Art.