250 L-6 to 350 Questions

[6 Door King]

Before anyone directs me to a link within the site, I DID search it and only got returns for older model trucks and my questions are more geared toward motor mounts and engine crossmebers.
1st, INFO: I have an 84 SWB that I am working toward comsetically restoring so I can flip it for a quick profit. I plan to swap out the 4.1 for a 350 from a K30 that is supposed to be turning 400 ponies at the wheels in order to help the truck sell.
Question is this, if my motor is coupled to a 3 speed auto, (not sure if TH350 or 400) and the donor engine was also coupled to a TH350, will my flywheel on the new motor need to be changed? I have heard there are several different patterns.
Second, will the crossmember and engine mounts need to be swapped out? Obviously the engine side of the mounts will be different, but are the mounts on the frame the same and just in different locations like the 67-72 models?
Thanks in advance guys, hope you can all make sense out of my plans and questions.