I have their frame on my 69 Camaro. I have 275/35/18 tires with 9.5 wheel with 15mm offset which equates to about 5.75” offset. The only issue I have is the tire rubs the frame at lock to lock. I put a 1/4” spacer and it still rubbed the frame slightly so I had to put rack and pinion spacers to limit travel. I put 1 spacer on each side now no rubbing.
So with the 1/4” spacer my offset is 5.5” or 10mm. So I think your safe with 5.5”-5.75” of offset. You will definitely need the rack spacers. BTW, I have stock inner fender bolts and the tires do not rub on the bolts.
Here’s a link to the spacers.
1979-2004 Mustang Steering Rack Limiter Stop Spacers - 6pc
https://a.co/d/gYb5BLz