Thanks for clarifying the position on the manifolds Simon - i've been looking at a set of OBX stainless manifolds for my Sterling and the original Lambdas are in the cast iron "logs" bolted to the heads with a downpipe flange stuck to the bottom of each log. Just above this flange is a Lambda boss in each.
On the OBX manifolds/front pipes, the rear bank Lambda is about 6-9" (rough, edumacated guess) from the "Y" join between front and rear bank downpipes and the front one is probably about a foot, maybe 15" from the "Y". This means both senors are a fair distance from where they were originally and would almost certainly need to be changed for heated sensors (unheated, single wire sensors as standard) which isn't a problem in itself, easy to enough to pick up an ignition switched feed, put it onto a relay coil to supply the heaters but i suspect that not only will there be a delay in mixture control due to the distance but because the front run is about twice the length of the rear, it could be that each sensor is seeing a different air-fuel ratio.
Nothing like the problems you report on that Vortec V8 and only one pair of Lambdas to worry about (no post-cat sensors on mine) but still enough of a potential problem to be worked out and solution found before shelling out $$$$$ on some nice, shiny header pipes!
You mention overpipes on this V8 you've been sorting, that would be the ideal place and very easy to do for a pair of pre-cat Lambdas. You probably already know but you can get Lambda repair sections that can be either clamped or welded over the original pipework :http://www.ebay.co.uk/itm/281128311754? ... EBIDX%3AIT
About 6" long, this would probably solve two problems in one go for you, different diameters are available as well. On my (recently acquired) Volvo V6, where the front pipes join the front box there is a flange separating the two assemblies. There are blows both sides of this flange so no prizes for guessing how i'm going to stop them blowing! So far the front pipes seem to be NLA although the front box is available. Even if i got a new front box and used those Lambda repair sections to fix the blow on the engine side of the flange i still get to kill 2 birds with one stone.
I'm guessing the overpipes on your V8 would have to come off to weld a Lambda boss on, these would probably pay for themselves in terms of time saving and renewing for the customer althoughi'm sure the customer wouldn't mind paying for them. Not sure if the next bit would be possible but i daresay once it had two working Lambdas, you could link the pre and post-cat Lambda inputs to fool the ECU into thinking it had 4 working Lambdas - some ECUs measure the resistance of the heater circuit and/or the sensing circuit so this may not work - worth a try though!
Somewhere in Suffolk with a Jeep, 2 Rovers and a V6 Volvo