He's looking at either a 350 Chevy or a 302 Ford, there's also severail other freinds trying to talk him into putting a V-10 out of a Ram in it.....

There is a guy on NAXJA that put a dodge 318 into his xj. Might try looking at that thread for some info.

What aspect of the swap would you like to know?

-is it difficult?.....NO and depends which mount Mfg. you use (don't use Adv.Adptrs;all the wrong design for a XJ/MJ...lets just say barily adequate for street use)
-Is it pricey?....Depends what you are comparing it to

- the 4.0L is a great motor but it does lack what a V8 (stock) can provide

overall this swap must be considered like package. main points to be changed:
-the sus' has be changed to a radius long/arm...should go this way regardless
-cooling system changed---4.0L has a hard enough time staying cool 
-if money is the primary concern...here is the biggest money time/saving advice for the swap ....**change out the entire drivetrain, motor,trans,t-case. sell the old (using adapters are hack and will cost more time and money than the new V8, and the stock drivetrain is inadequate for a V8).