With the catless headers being significantly more compact, bolting them on was a fair bit easier since they slid through easily enough so that the engine arm didn't need to come out again. I also replaced the stock studs with new ones from the N-series engines that are slightly shorter and have an E-torx head, making installation a lot easier than the double-nut method.
I thought about reusing the manifold gaskets with the heat shields, mostly to retain the clips for the O2 sensor wiring but due to the different positioning of the bungs those wouldn't have worked anyway. The new headers are angled straight down, are quite a way away from the valve cover so keeping the heat shields probably wouldn't have had much effect apart from making access to the bolts a lot more tedious. Opted for brand new M52 gaskets that are pretty much identical to the M54 ones, minus the heat shields.
The bank 2 O2 sensor (990mm) had enough length in it to fit into the factory clips, bank 1 however (570mm) was now considerably further away and wouldn't reach at all. The options were to order in a new longer bank 2 sensor (which might still have been too short) or make up an extension which I decided to have a go at. Not the first time the spare wiring loom has come in handy. Hacked it up a bit more, as well as a spare old O2 sensor and made up a nice factory-looking extension that blends right in.
I only recently discovered these heat shrink + solder butt connectors and now swear by them. So easy to use and seem to create a nice, solid connection. 10/10 recommend.
The additional new connection didn't fit under the beauty cover but slotted in between the valve cover and the fire wall and sits there quite securely.
The other issue with these headers was that the bank 2 O2 sensor bung is positioned so that the top of the sensor pretty much sits right up against the fire wall heat shield. I bashed them in at that spot as much as I could but it's still mighty close. Not ideal but it is what it is.
The exhaust was a bit tight going on but the flanges bolted up fine in the end. The bank 1 flange must still have ended up angled slightly wrong with a slight gap over top of it but luckily the edge of the inner opening looks to be slightly bulged and the gasket looks to be sealing fine.