And yuu'd becadvis3d to keep a gparted/similar disk at hand for when you find (through some unknowable future quirk of system demands) that the 25%/75% split (or vice-versa) is now thottling your expanded usage of the (usually!) smaller partition, and you need to shift the 'hard boundary' around to accomodate extra installs/data how you'd prefer...
(Not that it helped when I had my computer with two
physical 1GB disks... Win95 days, this was. No amount of regular repartitioning could seemlessly lend unused C: space to a nearly filled D:, or vice-versa. And was before more virtualised solutions and too much fuss to try a RAID-like recombination of drivespaces.)
TBH, I was imagining "new machine, new OS" (or install licence, at least). The ability to do Whole Body Transplants is severely restricted by the antipiracy measures MS (I assumed Windows, naturally, anything from XP onwards) has enforced in its systems, never mind the significant driver changes. OEM tools to 'denature' an install (before then re-registering with the approved registration key/etc) always used to be a key part of gettingblegal (and possibly even working) clones. Not see how post-Win8 does it, but never seen any reason to expect relaxed requirements trickle-down from MS. For one thing, the highly deprecated offline validation (8n today's xalways plugged in" world).
As such I was (at the beginning at least) assuming the donated drive would just be a legacy-data carrier once the new machine. For which my advice would be: Physical connection should he easy (with the caveat on the no-IDE headers possibility, if it's an IDE donor drive) and make sure it doesn't usurp Boot Drive position. And all that stuff about non-trivial (e.g. password) data migration that you now need to know you should look at before dismantling/remantling.
If you're doing the WBT (whether all-but-the-disc changes or all-but-the-
data...) then perhaps the non-trivial data is not one of your worries, as it is actually more intrinsic to the 'mind' you're transfering between systems. But backup (or copy across to new disc and have original as your backup by default, only to overwrite once there's clearly no further issues arising) would be advisable in one form or other. IMO.
Truly, though, the difficulty is not in the doing, but in choosing which exact method of doing is best suited to you, to avoid the pitfalls you are least capable of overcomng. And for any three IT professionals, you'll find at least seven different opinions on how
they would set about it.