The information in that ECGS link is annoyingly incorrect. According to a post by Carl Jantz, the pinion offset for the D61 is 15/16" vs 1/2" for a D60. In other words, the pinion sits 7/16" farther away from the carrier flange on a D61 vs a D61. I am doing a D61 front right now (got it cheap, which is the big benefit
), and I checked the offset for myself. It was hard to get an accurate measurement, but I got .425" which jives with Carl's claim of .437". Everything on Dana axles uses common fractional increments, so I assume .437" (7/16") is indeed correct. ECGS claims 5/16" (.312") or 5/8" (.625"), neither of which are close.
A D61 with the 3.07 gearset uses the exact same part # carrier as a D60 4.10-down carrier (706040X). D61's with 3.54 and 4.10s use the same gearsets as a D60 (706033-1X and 706033-3X respectively) but with a special D61 only carrier that moves the ring gear 7/16" closer to the pinion (706400X).
So, to regear a D61 to "D60" gears, you have a few options. To do so without spacers, you can use the 706400X carrier to run standard 4.10-down gearsets and THICK 4.56-up gearsets. In order to use a standard 4.56-up gearset with the 706400X carrier you need the readily available .160" spacer made by Mr. Gasket. Since the pilot surface on the carrier is substantially more than .160" thick your ring gear will still be properly located on the hub.
If you want to run a D60 carrier (a locker for example), you MUST use a ring gear spacer. Here you have two choices: a 4.10-down carrier and a 4.56-up carrier. If you use a given ring gear with its normally matched carrier like you would in a D60, then you need a 7/16" (.437") thick spacer. In order to use a 4.10 or numerically lower gearset with a 4.56-up carrier you need a .277" thick spacer (.437"-.160"), while using a 4.56 or higher gearset with a 4.10-down carrier will require a .597" thick spacer (.437"+.160"). Using a thick 4.56-down gearset with a 4.56-down carrier will require a .277" thick spacer (.437"-.160"). I am using the .160" difference between .410-down and .456-up carriers from the thickness of the Mr. Gasket spacer. I have never actually measured and confirmed that value. The only measurement I am sure of is the .437" difference between the D61 and D60.
Any of those spacers (.277", .437", .597") will be thicker than the ring gear pilot surface on the carrier, so the spacer itself will need to pilot on the carrier and then have a concentric pilot surface for the ring gear. Reider discontinued their spacer and as far as I know NOBODY else makes spacers in any of these thicknesses (except for the .437" thick one I am making for myself :grinpimp
, so you would have to have a local machine shop make you one. Since the pilot ring on the spacer needs to be stepped inwards you will also almost certainly need to do some minor turning of the carrier for clearance (this is the case on mine - it isn't much and is easy to to though). Finally, you will need bolts of the proper length. Luckily, ARP started making generic 1/2" fine thread bolts in 1/4" length increments last year, and combined with their 1/8" thick precision ground hardened washers (if necessary) gives you a selection of high quality 180,000psi fasteners in effectively 1/8" increments. In my case, to run 4.10's on a 4.10-down carrier with a .437" thick spacer I will need 1-3/4" long bolts with a hardened washer under each one to get the same thread engagement as the stock bolts with no spacer. As a bonus the fasteners are 30,000 psi stronger than the stock grade 8 units, and the extra 9/16" of non-engaged bolt shank will give more consistent preload.
Finally, some people are squeamish about running ring gear spacers. As long as the ring gear is properly piloted, it is just as strong as running with no spacer. The bolts are not, AND SHOULD NEVER BE, loaded in shear. All torque is transmitted through friction between the tightly clamped ring gear and carrier flange. With a spacer, there are two interfaces, but each is as tightly clamped as a single interface - the clamping force is NOT divided between the two as some people think. A single 180,000PSI 1/2" ARP bolt develops 16,391lbs of force when torqued to it's 75% yield point. Since there are 12 bolts attaching a D60 ring gear, that is a total clamping force of 196,692lbs or about 98 tons. That ring gear ain't going anywhere
PS: Elwenil is 100% correct: With the exception of the pig and the ring gear, EVERYTHING on a Dodge D61 front axle is identical to the same year D60 front.