So, on the bank holiday monday, I did a little more measuring and
confirmed that I should be able to get the manifold as-built to fit
under the engine cover with a bit over an inch of clearance. Satisfied
with this, I committed to more welding!
And a bit more...
And then I ran out of welding gas again. This is really getting on my
tits... 
Thankfully, I have an early finish at work this Friday so that I can
go to a medical appointment (More testing, hooray!), so hopefully I
can nip into the supplier and swap the bottle afterwards.
Going back in time, though, I figured I might as well pick up where I
left off in the nearside of the cab. So, chop the rot out of the
seat-base, being careful not to hit the data-plate and whatnot, and
start snotting in fresh metal. (It'll clean up with some grinder
action, no worries)
Zap zap zap...
Upright panel trimmed (slightly too short in one spot, of course.) and
folded over to be welded in. Discovery of the day, that Zinc-182
primer smells like rotten milk when it's burnt by the heat of welding
next to it. Yuk!
Decided I'd had enough of that, and went to go refit the wiper mechanism. Still need to re-tap the switch standoff to replace the screw that holds the cover on the wiper motor, but I'm waiting on the tap for that and there's no real rush. It'd just be nice to have it not disassemble itself while I'm trying to work the switch.
The good news is, with the spindle arm shortened as much as I have it,
I do have the sweep I was after!
The wiper shown here at its start position, and you can -- hopefully
-- just about see the end of the sweep in the dust on the screen
(About in-line with that funny yellow and green crate)
Unfortunately, it sweeps out to its end position, and then it all starts going wrong. There's enough drag -- and the lack of leverage afforded by the shorter linkage arm -- that the wiper blade can't fully return to its start position, because the long linkage rod just buckles and effectively becomes shorter. It might not be a problem with water on the screen, because that might reduce the friction enough to work out fine. If not, then I guess I've got to make a slightly more rigid linkage rod.
While I was pondering on that, I went and repurposed a hole in the
dash for a fan switch. Nothing for it to connect to, yet, but I'm
tumbling ideas around for a compact heater box in the dash for
windscreen demisting purposes. (Shouldn't need a heater for the
occupants of the cab, from what I've been told of how much heat comes
up from the engine hump anyway. :D)
On the fuel tank side of things, I've got my big holesaw, so I'm almost ready to go with that; but it will mean driving the truck outside to take the tank off. (Which means moving a bunch of stuff from under the truck again, and rigging up a temporary fuel system again.)
OH! That reminds me: The fuel filter housing I've had sitting around,
that I'm planning on using, turns out not to have NPT ports but
1/2-20. I can adapt it to what I need, but that's just funny to me,
especially since the inlet on the carburettor is also 1/2-20. 