I've had a 66 straight axle car for 26 years and started the process of redoing the car five years ago. Bought a MAS axle first and ran into problems with king pin fit (too tight) and deflection (with the weight of the car-motor on it). Then bought the speedway kit. Same problem with deflection (about 3/4 to a degree of camber) This is with a big block with all aluminum parts so an all-iron small block wouldn't be much different. Then had a local firm make two more that they couldn't get the camber right and had the same problem with deflection (all these axles are 2" mild steel 1/4" wall steel) I also wanted the axles wider by 4" (the widest Speedway/MAS/Bell/ect makes is 48") because that makes the track comparable to the stock Chevy II width. Yes, I could have compensated for the deflection by having the builder alter the camber setting but the point was to not HAVE the flex.
So then I bought the Tom Medlock-Bill Thomas copy of the instant funny car kit. Tom sleeves the inside of his axles with another piece of stock, so that took care of the flex. But I was using the Speedway forged chevy spindles and Medlock only uses Ford ends, so there went about 600.00 out the window. The better part of that is there is a much broader (and lighter) brake-spindle selection for the Ford spindles than the Chevy. I sent the axle back to Medlock and he made me one wider, But I had him leave the pin ends tight so I could finish them myself. Most finish the pin ends too loose, and I didn't want the pins to oblong the ends (it happened to my first axle. I ended up not using anything else from Medlocks kit because I wanted to use my stock frame, and I didn't like his springs or mounts. Most of these kits use short boat/RV three-leaf trailer springs and you want to stay away from those. The six-leaf Speedway ones are automotive application, and the best by far.
The Speedway axle-to-spring mounts were the best as they were the best constructed (more area to weld to) and you could use U-bolts that clamped the spring, instead of the tube. Also, nobody offered any lower shock mounts that were little more that two tabs. I found a one-piece mount from an air ride company that resembled the shock mounts on a 70-80s 4WD Dana front axle. Looks much cleaner as well as being stronger. The optimum setup is to have the shackles in the rear (better geometry) but I didn't like the angle they would be on as the frame takes a downward turn at that spot, and I've seen them break from coming down hard mounted that way.
Using the stock steering box (what I did) cuts down on fabrication, and steers just fine. use a power-steering pitman arm so you can bolt thur it. it's not a good idea to bend the arm as it's cast. Cross steering is more prone to bump-steer so run the drag link to the drivers side. Rule of thunb is to keep the drag link level with the car sitting on the ground. Drag links on angles promote bump steer (see any 4WD sites about this). They also sell small rubber dust covers for the heim ends to keep the dirt out.
I also just found a motor motor-crossmember that will work with a straight axle setup. Most all motor mount-crossmember mount directly under the center of the engine mount. This won't work as it's going to interfere with the upwards travel of the axle. Chassis Engineering (street rod) makes a tubular mount that offsets the engine mounts to the rear three inches. This facilitates eliminating the strut rod crossmember and makes a stronger motor mount. I looked forever for one of these (a defunct company called PSI used to make this type of setup) and it looks period-correct. If you need more info or links, let me know.
