Those are the rear coilovers. I checked that website you linked and there doesn't appear to be a front BC Coilover in their ER series? At least not on that website.
I've been looking into purchasing the BC Racing extreme low kit for the 350z. The shock absorber assembly length is 2" shorter than other available 350z front coilover assemblies. That brings the shock assembly height closer to the stock Thunderbird, so no need for cutting and welding and taking the risks. I guess it would just be applying the already aforementioned steps needed like enlarging the lower clevis mount holes and modifying the upper mount to take the BC coil spring. You can also purchase the necessary individual parts from the BC racing website if you're in the US. They also offer the ER series kit that seems to have better dampening but its like 200$ more per coilover. I know BC is on the upper end of midrange, lower end of high range brands, but for regular everyday use it should suffice.
Cut and weld?Right, it's just that there are almost certainly better options, dimensionally speaking. A shorter fork will allow for a taller shock body.