I get that you dont want to do it yourself, fair play; no big deal
But those talking of safety, how much more safe do you think a local garage are going to make it..?
Fuel tanks etc...aside, they're going to drill the same size holes as you would, use the same sized bolts and same size spreader plates (you'll probably have to provide said items...) so unless they have some re-enforced pixie dust they can throw on for good measure, again...how much safer do you think they'll make bolting a cage into the floor..?
I'm not knocking OP for wanting someone else to do it for him,( I wouldn't have a job if everyone wanted to do everything themselves...) but unless you are having a crash tested system professionally fitted (and I'm sceptic on those) then the safety arguement is balls