The distributor must be taken out. Just use some way to ensure that you replace it in the same position you removed it. I mark the rotor in relation to the dist body and the body to a point on the engine.
I agree with Forty9 about the markings. Also I believe as you pull out the distributor the rotor/distributor shaft will rotate some as the distributor gear comes up and out from the camshaft. Make a mark here also. You need this to go back in exactly as removed so distributor shaft lines up with oil pump shaft. Maybe use your phone to video how things come apart.
x2. I take the distributor cap off and rotate the motor until the rotor is at 12 o’clock to the motor. I mark the distributor body right at the lip where the rotor is pointing with a paint marker. Pull the distributor, with a black magic marker, scribe a line at the top of the clamp all the way around the distributor body shaft. The reason you want to do this is that it is entirely possible to get the distributor installed and the motor running, without the distributor tang engaging the oil pump slot. So the black line helps you confirm the distributor is in at the same depth as it was when you took it out.
install the new advance On the distributor, leave it loose, install the distributor, install the hold down bolt and center the octane selector to 0. body and rotor pointed to 12 o’clock, tighten the clamp screw. You have this!
If the distributor doesn't drop all the way in, never fear, the oil pump drive gear just has not dropped in yet, do NOT hammer on the distributor, just turn some more until the oil pump drive drops in the slot. If one looks at it, one can figure out on your own.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires. '47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle. '54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed. '55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
I do everything involved with the distributor by starting with aligning the timing marks to no. 1 cylinder. Check the position of the rotor, it should generally point to the 5:00 o'clock position. Disconnect the vacuum line. Pull the cap out of the way. Remove the hold down bolt and loosen the screw holding the advance to the list. shaft. Slide the list out. Remove the old advance unit. life the new one on. re insert the dist. in as close to the same position as possible. If the List. doesn't go all the way down, back it off and turn to rotor one way or the other to see if you can get it to easily slide in and retain the 5 O'clock alignment to the Distributor. body. This may take several tries. If all else fails, jog the engine until the shaft slides in and you have no.1 cylinder and the rotor aligned to No.1 cylinder at the 5 o'clock position. Install the bolt, align the octane lines to the center mark, tighten the screw but not complete yet , and install vacuum line. Static time the engine by hooking a test light to the Distributor side of the coil. turn on the ignition switch, adjust the distributor by turning it until the light just comes on. Lock it down. start the engine and use a timing light to get the final timing perfected. Insure to time the engine in accordance with the recommended RPM.