I didn't measure before and after but a quick comparison of each mounted tire shows about 3/4" difference.
So each inside dual is now 3/4" closer to the rear springs, close but not too close.
Overall width measured across tire sidewalls is about 1.5" wider with 8R19.5 tires on 6" wide rims than the width with the stock 18" wheels/tires.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-