Which K member?

Depends on the year of the car.. 67-72 used the same style mount (biscuit) setup, and 73-76 used a spool mount - to me using a spool mount K is more work than it's worth.. I would swap to the earlier K - be it stock or aftermarket (I went from a slant K member to a QA1 small block based K member)

No do not need to change K member if 67-72 - just need SB to BB swap mounts, mount shims, and a torque strap

Trans-dapt mounts are $60 - still need shims below
Summit fox mustang torque strap for aftermarket K members works after making a 2" longer metal tab to mount to the motor mount tab on the engine($60)
Schumacher mounts are $160, shims $45, and torque strap for stock K is $120
Stock buscuit style K probably get for $50 I'd imagine
QA1 K memeber for stock suspension $587

Spool mount requires cutting and moving the frame mounts to line up with the BB..

OR a motor and mid plate if you like to party LOL