4l to 4.6 gearbox question

Hi all, hopefully, someone knows the answer to this off the cuff.
I need to replace the gearbox on my 2001 4l D2. A little further down the line, I want to top hat, and convert to 4.6l. I haven't really looked into everything that involves yet, but my understanding was that you get the block top hated, new rods, crank, then you rebuild with the old spec parts affectively.
But would you then need to mod the gearbox,? I know the D2's in America with a 4.6 had a slightly different zf gearbox model number, so didn't want to potentially spend 1k on an ashcroft box now only to have to change it when I come to do the engine.
Any help appreciated, I have asked ashcroft, but they seem pretty busy (or there tec people are), and I'm not 100% sure it's for them to know as opposed to for me to find out?
Many Thanks
Matt

For anyone interested. Ashcroft ok'd this.
As its 15mm longer, you have to drill the mounts (assume the t-box), remove the spacer between the flexplate and starter plate, and you are good to go. Plus check rear prohaft doesn't bottom out.
He also said it may be a little sluggish on pick up in 4l form, so guess that will nudge me closer to going 4.6

As you are going to convert their 4.0 into a 4.6, it is still going to drop back in without any gearbox mods. The 4.0 used a 4HP22 and the 4.6 a 4HP24. The only difference is the 24 is designed to take a higher torque load than the 22. However, you would have to be doing a lot of towing or heavy work to kill a 22 with a 4.6.

My advice would be to do your conversation, use the original set up, if the box dies at some future point, worry about it then.

Thanks Marshall
I actually ordered it 10 days ago with the stronger 24 internals and Ashcroft told me it will be 15mm longer. Good to know though that when I do the engine it will just drop back in.
In hindsight you're way would have made more sense. Hopefully, the garage that is doing it will find it straightforward.
Regards
Matt