Donor for 63 dart GT

Is it a bolt-in? No. Is it at least do-able? yes, depending on your abilities and ambition.
There are many differences that will need to be addressed: Engine, transmission, rear end, suspension, and electrical.
Let's start with the engine.
The '75 engine will physically fit, but the following will need to be done:
You will need to use the engine mount brackets and insulators from your '63 on the '75 engine.
You need to use the '63 oil pan, pickup and dipstick; the early pan is slightly different to clear the early steering centerlink.
The throttle linkages are different, the '63 uses a rotating throttle rod and the '75 uses a throttle cable. You can work around this a couple of ways, either swap the intake and carb from the '63 onto the '75 motor, or swap the '75 pedal and cable into the '63 (not a direct bolt-in, some adapting is necessary. Easiest would be to locate a '64-'66 V8 throttle pedal assembly, which would bolt in to the '63 and uses a cable).
Ignition is different; the '63 uses points and can be swapped onto the '75 motor, or the '75 can donate parts of it's wiring harness, ECU and ballast resistor to retain the '75's electronic ignition.
Similar situation with the charging system; the '63 uses a mechanical regulator and single field alternator; the '75 uses a dual field alternator and electronic voltage regulator. Either retain the '63 parts or modify the wiring to update to the '75 parts.
Cooling system: you could retain the '63 radiator, or adapt the '75 to fit- but depending how it's optioned, the '75 radiator may require modifications to the '63's radiator yoke opening and radiator's mounting brackets.
Brackets, pulleys and accessories will need to be determined depending on how the two vehicles are optioned.
There will be other changes needed, I'm sure, but that's what occurs to me offhand.

Now the transmission.
You didn't specify what the two cars are- is the '63 a floor or column shift? Assuming the '75 is a floor shift, but is it a 3 speed, four speed or OD? The particulars of the swap will depend on the answers.
All will physically fit your '63, although any of the floor shift transmissions will require cutting the floor pan and installing a transmission "hump" to clear the linkage and shifter. '64-'66 cars (which would fit your '63) had different style humps than the '75, although it may be able to be adapted depending on your abilities and level of creativity.
Clutch linkage- The two years had different width frame rail spacings, so the torque shaft (Z bar, bellcrank, whatever you want to call it) is wider from the '75 and will require a bit of narrowing to fit the '63. The pivot locations on the frame and bellhousing also differ between the two years.
The transmission/driveshaft on the '63 uses a ball and trunnion front joint, whereas the '75 uses a splined output shaft and slip yoke arrangement. You will need to use the '75 style shaft, although both cars share the same 111 inch wheelbase, the driveshaft lengths are going to be a bit different due to the engine mounting location, so plan on having the '75 shaft shortened or having a new one made.
You will need to use a '66-'67 speedometer cable to connect the '75 transmission to your '63 speedometer, those are the only two years that have the correct fittings on both ends of the cable for this swap.

Rear end.
The '75 is *most likely* going to be a big bolt pattern 7 1/4. Fundamentally the same as your '63's small bolt pattern 7 1/4, pretty much a bolt-in with the possible exception of your parking brake cables; although you should be able to use the '63 cables or mod the '75s to fit. This will also give you 10" drums in the rear, an upgrade from the '63's 9" stockers. The biggest change is that you'll now have a 5x4.5" bolt pattern in the rear, which doesn't match your 5x4" pattern in the front. Which brings us to:

Suspension and brakes.
If the '75 rear leaf springs are in better shape than the '63, swap 'em in- they're the same size (the front and rear spring hangers are not the same, however).
Same goes for the torsion bars in front- they'll interchange if needed. But to get the front bolt pattern to match the rear, you'll need to do some swapping with the front brakes, too. With any kind of luck, the '75 has front disc brakes. To use them on the '63 you need to swap the upper control arms, spindles, lower ball joints and brake lines. Use the master cylinder, booster (if so equipped) and proportioning/combination valve from the '75. If they're still in decent shape, you may be able to re-use the appropriate brake lines from the '75 to aid in the swap, since the plumbing part of the brakes is totally different due to the double reservoir master and combination valve.
If the '75 has front drum brakes, you can still swap them on (same procedure as discs, above) and gain the 10" brakes, not as big of an improvement as discs, but an improvement nonetheless.
If the '75 has a front anti-sway bar, you won't be able to use it on the '63, as it will be too wide as well as being of the type of design that goes "through" the K frame rather than under it.

These are just a few of the basics, I'm sure others will chime in with any other details or things I may have missed. And a little clarification is needed as to the transmissions in the vehicles.