I'm planning on kds mounts...Car has a 6.0 swap already...But has a modified truck pan that has seen better day's...Transmission is from the donor truck also..Oil pan is close to suspension, and transmission crossmember hole has been slotted for trans mount to line up...Almost like engine sits a few inches forward...Hoping kds mounts can remedy this?..
If you are planning on swapping to KDS mounts, the mounts were designed to work with the F body pan so that will work fine. The holley pan also fits so it's just a matter of preference at that point.
Just keep in mind that a lot of LS swaps I've seen with adjustable mounts place the engine a decent amount forward from where the factory drivetrain would have sat. The KDS mounts retain the stock drivetrain angles, engine placement (front to back) crank centerline height I believe which to me was a big selling point because I don't like the idea of moving the single heaviest component of your car even an inch more forward or higher than where it was before. Issue with this is if you swap mounts, there is a good chance your current headers may not fit and you may run into some other minor clearance issues from the change of positioning.
As far as the slotted X member goes, this is more than likely because of the mounts you currently have, and with the KDS mounts you should be able to run a factory crossmember IF you are still running a Gen2/LT1 4l60e. I swapped over to an LS 4l60e and the trans mount position was WAY further forward than the LT and I had to do some extensive modifications to my tubular X member.
Another note, KDS mounts provide 3/8ths of wiggle room by having slotted holes to account for chassis to chassis variations. To clear the F body pan I had to run my motor in the furthest to rear position. Aside from some clearance issues I had to address with my LSA supercharger and tall style valve covers, everything fit fine and this actually helped give me a little more clearance to the K member of my 1 7/8ths TSP headers.
Depending on what headers you have now, it may just be easier to find a pan that works with your current mounts rather than risk running into header clearance issues and possibly AC compressor clearance issues by swapping to KDS mounts. KDS mounts work great with a very specific part selection (F body pan, F body headers, truck accessory drive) but if you don't already have all of that, it could be troublesome to switch over to them. I can't remember if a truck AC compressor clears without notching the chassis either, you may have to run a mini compressor.