Here is my write-up on converting a classic Mustang from 6 cylinder to V8 components.
1. Front end
The stock 6 cylinder front end has light duty spindles. These must be replaced with V8 parts. There are several options available for this.
The strongest option is to go with 70-73 Mustang spindles. They can be bought new at several places below:
You can also go with spindles from a ’70-’77 Maverick or Comet, ’75-’80 Granada or Monarch, or ’65 – ’73 Mustang.
Remember to use the outer tie rods that match the spindle you are using.
You can leave the stock 6 cylinder coil springs, they have a lighter spring rate so they will lower the front of the car. However, you may want to upgrade to a stiffer spring. These are available in different spring rates from almost every Mustang part supplier.
While you are going to the trouble of replacing your spindles, it is a good idea to upgrade to disk brakes. There are several conversion kits available. Remember to use the kit for the spindles you have (ie 71 Mustang spindles on a 65 Mustang, use 71 Mustang brake kit) Here are some links:
When upgrading to disk brakes, you might have to use 15″ or larger wheels to clear the calipers. You will also have to redo your brake lines and master cylinder to accommodate the proportioning valve.
The stock six cylinder steering is different than V8. You will need new inner tie rods, adjusting sleeves, center link, idler arm, and pitman arm. These parts can be bought new from almost ever Mustang part supplier.
While you are upgrading the steering, you can install a Shelby “Quick Steer” kit, which consists of a longer idler arm and pitman arm. This gives a quicker steering ratio. This kit will not work with power steering.
You can also upgrade to a roller bearing idler arm for greater ease of steering.
The transmission that came with the 6 cylinder can’t handle the torque of a V8, so it must be replaced. Now would be a good time to consider a modern AOD or T5 swap. The driveshaft must also be replaced with a V8 unit.
3. Rear End
The stock 4 lug rear end will not hold up under the power of a V8. You have a few options for replacing it. You can get a stock 8″ rear end from a V8 Mustang; you can get a 9″ either new or from a car with the same rear end width; you can modify an 8.8 Ford axle which usually comes with a better gear ratio, traction-lok, and disk brakes; or you can try to find a Lincoln Versailles rear end, which has disk brakes. Remember that you can only have rear disk brakes if you have front disk brakes as well. There are several articles concerning 8.8 swaps online.
Swapping an 8.8 into a ’65 – ’66 Mustang can be accomplished two ways. You can cut the driver side axle tube down to fit a passenger side axle, thus narrowing the rear end to fit like stock. Or you can leave it as is and get custom wheels with more backspacing. If you don’t shorten the axle, you may need to modify the rear passenger floor to give clearance for the driveshaft when you hit a bump in the road. Another thing that must be done when swapping an 8.8 is to move the spring perches. The leaf springs are farther apart on a Mustang than on an Explorer. You can add on to the existing perches or cut them off and weld new ones on. If you weld new ones on, be sure to set the pinion angle correctly and make sure everything is even on both sides. The factory pinion angle on Fords is 5 degrees up, but you may need a different angle depending on ride height, transmission angle, etc. There are many articles on setting pinion angle online.
When swapping to a V8 engine, you will need to install a larger radiator and V8 engine mounts.