Re: Battery cable routing from house battery to starting bat
On my current (although non-SMB) E-350 (and on a friend's other-brand E-450), the cable runs "up high" across just below the hood seam (just below the windshield wiper grille things on the firewall), then down near the steering rod thingie (yes, I'm just full of technical terms today) and then back along the driver's side frame rail (where it goes back up into the coach, but in my case that is not through a stock van body). That might work on a van body as well though (at least the first part, if nothing else).
I didn't ever figure out the exact path of the SMB wire that ran aft, but it looked like it might have gone through the firewall and under the driver step-well cover?
One thing you might want to check is if the ANL fuse has enough AIC (ampere interrupt capacity) for your house bank. I see they are rated at 6,000 AIC (at least the ones on Blue Sea's website) and, depending on how large/what type of house bank you have, you might be over that. For example, two Lifeline Group 31 AGM's together have a short circuit rating of over 7,000 amps; two Llifeline Group 27's would run about 6,750 amps. The Blue Sea MRBF's are rated for 10,000 AIC. I have three Lifeline Group 31's (rated at a combined ~12,750 amps at short circuit), so even the MRBF wasn't quite high enough and I went to a Class T fuse (20,000 AIC) instead. Still at 250 amps.
(If you haven't used AIC, what guards against is, if you have a short, that the fuse holder itself does not fail - thus rendering the whole idea of the fuse somewhat moot. Today's larger battery banks can really be large power storage banks. If you have a bank whose short circuit rating is under 6,000 amps then the ANL would be fine.)
BTW, I have found that on the recommended Group 65 battery and with the stock hood (tested a 1997, 1998, and 2003), a MRBF/holder (Blue Sea) will fit on the positive terminal and not interfere with the hood. What I did was get a so-called marine terminal adapter (it's like a battery clamp mated to a stud), then put lug terminations on both the Ford wires and my wire going aft to the house bank; the cable going aft to the house bank sits on a Blue Sea MRBF that is on the post - this all clears the hood. The 10,000 AIC of the MRBF is adequate for the Group 65 AGM starting battery, and then you don't need to separately mount a fuse (and then I have the Class T fuse on the house bank where the higher AIC is needed).
If this isn't helpful..... feel free to ignore! (I'm not exactly sure what your setup is.)