Yes you should run a return line for the FEL valve.
The old Part number (70060-00360)has been obsolete for many years, and the new part (70070-00385) has been obsolete for about 5 years, so the chances of finding one is going to be rough.
I haven't tried to find the return adapter but I suspect that might be getting tough to get too.
You can take the original block and add ports to the ends, then you will need to tap the cross over hole and install a cap plug then add a surface sealing plug to the outer plug to seal it (because when you tap the center cross over it will take the taper out of the exterior plug so the original plug won't seal).
Thanks Wolfman, Thanks for your advice regarding the return line, I think I've managed to find a NOS return adaptor so fingers crossed I can get that secured and shipped.
With regards to the Diverter block, given my lack of tooling... It might be easier for me to take the dimensions to a machine shop and get a new block milled and drilled.
Although hows this for an idea... I had considered seeing if I could get my hands on the complete high pressure line from a wrecked machine... this would come with the original block... then if I cut the pipes off either end and drilled and tapped the holes left where the pipes entered and exited the block to accept Hydraulic fittings... I could then use this as the diverter block... (PIC Attached)
I would need to drill the threads out of the 4 corner holes so that the bolts would fit through and thread into the existing block.
Each side would then have an o-ring so not sure how well these would seal together? (perhaps I would need to remove the inside surface to the depth of the o-ring rebate to get the two halves to seal)