[john-sussex]

There are no worries in your conversion. P38 and late classics differ a little but will still fit. If you put a 'Classic' engine in a 'classic' then no problem. The main thing with the later engines is in the oil pump drive and on P38 engines of course have no dizzy, but change the front cover for a classic serpentine one and away you go.
I have a P38 based 4.6(an a bit!) in my LSE went straight in.

 

[john-sussex]

front of crank is longer on 4.0 & 4.6, but even those are same in rear

Forgot about that I have a spacer in mine. It is longer to allow for the oil pump drive in the later serpentine engines before the question is asked.