Hi from a newbie, also Offsets on 20x10 rims?

[UK 58 apache]

Hi Guys,
I've been lurking on here for some time, finally registered and thought I'd say Hi from UK.

I'm not sure if anyone can shed some light on a question i have,
I Recently bought a 58 stepside, its running steels, its on air and has a GM 10 bolt with 5 lug hubs,

I really want to throw some alloys on it and I've seen a few task force trucks over here in the UK with 20x10's on the rear.

The guys at a local hot rod shop said you can't put 10's on a 58 with air and a 4 link with a pan hard bar, as its too tight and rubs???

I've seen 20x10's on stock rear ends with leaf springs and also with a triangulated rear end, but has anyone tried/got 20x10's on the back of their truck with a parallel 4 link and running air?

If so, what offset/back space and rims are you running as the gap is so tight, there doesn't seem much room for mistakes.

Many Thanks
Karl