Ok, we need to figure out just what you are talking about. You have an early 3/4 ton WC-51 with the 8.75” ring and pinion third members right? You switched out the 5.83 r&p’s for 4.89’s then right. Good luck finding one set of 4.30 r&p’s let alone two. Your best bet would be to locate 2 good used 4.30 8.75” third members and bolt them right in. This ratio was used in all 1ton dodge trucks from 1940-1952-53. The DU route van also used this ratio. The side gear spline size was changed in the civi trucks from 1945 onwards so make sure if you purchase a post war third member that you have the right axle shafts to go with it. 3.91 and 4.10 ratios were also available, but those are very rare also. You need to keep in mind that in going with a lower numbered ratio you will gain more top end speed but lose power going up hills. Believe me I know. I am running 4.30’s in my 1/2 ton WC with 900-16 tires. I have a 25” 236 Flathead in it which is mostly stock. It cruises good at 55-60, but anything more than a moderate hill and it losses pulling power quickly. Those 900-16’s are a heavy tire and wheel. Things to think about when switching out ratios in these old trucks.