I am getting ready to pull the trigger on rims, tires, and build a D60 now too.

The question I have, do I buy the wheels and tires first and build the axle to them?

OR

Do I build the axle first and measure for rims and tires?

I look at it the opposite way. Find the tire/wheel/backspacing combo that will fit and build the axle accordingly.

Wheels & tires first, then rearend measuring.
Learned that myself, and my racer-fab buddies do it that way.

Wheels and tires first as long as you are going to move fast enough to complete the build before the tires dry rot and the wheels go out of style.

When I mini tubbed my Dart I bought the wheel with the offset and tire I wanted to run. Then I made a fixture out of angle iron to hold the tire/wheel square and used a floor jack on each end. I raised the tire/wheel into the wheel well to the approximate ride height, then centered it in the wheel well. I measured inside to inside, deducted the rotor thickness and used this measurement to order my rear end. No way I'd order the rear end first.

I bought wheels and tyres that I no longer like years later. Nothing like a divorce to slow things down!

I like his way. When I changed axles I used my current wheels but went a tiny bit narrower because I needed the room. One thing he mentioned that I almost got bit by is the rotor thickness adding to the track width. On a drum brake it is almost negligible. The wilwood discs I was putting on with the axle swap have aluminum rotor hats that are near 1/2" thick. That would have totally wrecked my plan had I not caught that!

There is a good calculator on Dr Diff's web page for axle width and I believe it also accounts for the rotor had thickness, if it doesn't be sure to calculate for that.

best fit all around will come from Wheels /tires first. Put them under the car and get a measurement between the two rims. THen as already indicated thats your total rear end width, including brakes. This also lets you make sure the tire your running has enough wheel tub clearance and you can address that now rather than later after the car is done. IF your concerned about the tires being "old" by the time the car is finished, buy some second hand used up tires for mock up. There are plenty of used tires out there for free or sometimes really cheap compared to new.

wheels first, then junk mock-up tires of the size you plan to use.
make sure your wheel wells/tubs fit your tires, and don't forget tire growth.
then position tire/wheel combo and measure for the width of the rear, subtracting for brake hat thickness as has already been mentioned.

ok will do....

Now the hard part, FIND what tires and rims I want.

I went with the tires and rims first as well and then built the D60