Appropriate Wheel Offset for 18" Wheels?
As MuuMuu mentioned I'm running Dr Diffs 13" Cobra style brake kit. I have the 73+ K, linkage, and spindles. With all of that I also have a Hellwig hollow front bar #55905, stock 73+ sway bar mounts on a set of stock, boxed LCA's, and magnumforce non-adjustable, bushed tubular UCA's. Calipers are rear mounted to avoid any interference with the sway bar. I originally tried to run stock UCA's, but they hit the rims at full lock and suspension droop, so I went tubular for additional clearance.
Personally, I would run the 73+ K and steering linkage. There's a couple of advantages compared to the '69 K. The spool mount engine brackets are a better way to mount the engine. It's not a huge deal and I wouldn't swap the K's just for that, but they are an improvement. The bigger deal is the sway bar. The 73+ bars are better for clearance pretty much everywhere. The outer sway bar mounts give better clearance, and the bar runs through the K frame, so no sway bar hanging off the front of the K. It also seems like the aftermarket sway bars for the 73+ K's allow for more tire clearance at full lock. The 73+ Hellwig sway bar is also $90 cheaper than the 67-72 version. The Hotchkis bars are reversed for pricing, the 73+ bar is an extra $30.
As far as 17x9's, I think you're going to have an uphill battle. The biggest thing is going to be getting them to clear the outer tie rod. Most 17" rim designs don't clear the outer tie rod, and that limits you to ~5.6" of backspace. Which means 255's would be as wide as you could go. To run 275's with the stock fender openings you need about 6" of backspace on a 9" rim. I only know of one guy that got away with running a 17x9" with a 6" backspace, and he later alluded to having clearance issues. He was running Cobra R mustang rims, Bullit rims are usually limited to about 5.6" of backspace.
The 13" rotors might be an issue too, at least with some rim designs. I know that with my RPF1's I don't have a ton of clearance even with 18" rims, because the RPF1 has a stepped shoulder design. I'm not sure the RPF's would clear the calipers on the 13" kit if they were a 17" rim. But, I do have a 17" spare that clears the 13" rotors and cobra style calipers. So, I would say most 17" rims would clear the 13" rotors and cobra calipers, but I would bet there are 17" rims out there that won't.
My front set up is 18x9" Enkei RPF1's with a 35mm offset and 275/35/18 tires. I have a 3mm spacer in there right now, and probably need a 5mm spacer, so I would say with the 13" Dr Diff kit an 18x9 with a 30mm offset would be ideal, at least for my car. You can run a little less, I haven't rolled my fenders yet, although I did push them out a bit with the fender to bumper bracket braces.
As MuuMuu mentioned I'm running Dr Diffs 13" Cobra style brake kit. I have the 73+ K, linkage, and spindles. With all of that I also have a Hellwig hollow front bar #55905, stock 73+ sway bar mounts on a set of stock, boxed LCA's, and magnumforce non-adjustable, bushed tubular UCA's. Calipers are rear mounted to avoid any interference with the sway bar. I originally tried to run stock UCA's, but they hit the rims at full lock and suspension droop, so I went tubular for additional clearance.
Personally, I would run the 73+ K and steering linkage. There's a couple of advantages compared to the '69 K. The spool mount engine brackets are a better way to mount the engine. It's not a huge deal and I wouldn't swap the K's just for that, but they are an improvement. The bigger deal is the sway bar. The 73+ bars are better for clearance pretty much everywhere. The outer sway bar mounts give better clearance, and the bar runs through the K frame, so no sway bar hanging off the front of the K. It also seems like the aftermarket sway bars for the 73+ K's allow for more tire clearance at full lock. The 73+ Hellwig sway bar is also $90 cheaper than the 67-72 version. The Hotchkis bars are reversed for pricing, the 73+ bar is an extra $30.
As far as 17x9's, I think you're going to have an uphill battle. The biggest thing is going to be getting them to clear the outer tie rod. Most 17" rim designs don't clear the outer tie rod, and that limits you to ~5.6" of backspace. Which means 255's would be as wide as you could go. To run 275's with the stock fender openings you need about 6" of backspace on a 9" rim. I only know of one guy that got away with running a 17x9" with a 6" backspace, and he later alluded to having clearance issues. He was running Cobra R mustang rims, Bullit rims are usually limited to about 5.6" of backspace.
The 13" rotors might be an issue too, at least with some rim designs. I know that with my RPF1's I don't have a ton of clearance even with 18" rims, because the RPF1 has a stepped shoulder design. I'm not sure the RPF's would clear the calipers on the 13" kit if they were a 17" rim. But, I do have a 17" spare that clears the 13" rotors and cobra style calipers. So, I would say most 17" rims would clear the 13" rotors and cobra calipers, but I would bet there are 17" rims out there that won't.
My front set up is 18x9" Enkei RPF1's with a 35mm offset and 275/35/18 tires. I have a 3mm spacer in there right now, and probably need a 5mm spacer, so I would say with the 13" Dr Diff kit an 18x9 with a 30mm offset would be ideal, at least for my car. You can run a little less, I haven't rolled my fenders yet, although I did push them out a bit with the fender to bumper bracket braces.