I'm fixing my 4.6 that has slipped sleeve, so I was planning on using a 4.0 block and swap 4.6 crank and pistons and use refreshed 4.6 heads. It looks like the 4.6 pistons are longer than 4.0 and will hit heads. Anybody else run to this problem? Should I keep 4.0 block with original crank and pistons and put 4.6 heads? Any idea what can be done?

 

Blocks are same, but 4.6 crank has longer stroke, so they made up the difference with shorter connecting rods & piston assembly, as long as you use 4.6 crank with 4.6 pistons & rods, you will be ok.
But I don't know if heads have same size combustion chambers or not, (never had 4.0 heads to compare with 4.6 heads) , safe bet is to use 4.6 heads as well,

 

The 4.6 heads are the same with the possible exception of air fittings, which you will need anyway if your Disco has SAI.

 

Are the cams the same in 4.0 and 4.6?

 

Compatibility-wise? Yes.

 

They are different part numbers and the 4.6 cam has a different profile (read: higher lift, maybe increased duration[?]). They are completely interchangeable although I don't know why anyone would put a 4.0 cam in a 4.6 engine. I have a 4.0 P38 engine with a 4.6 cam installed and it runs just fine.