Whether or not you can use the timing pin depends on how old your 300T is.

The earlier version of the engine used tappets that were cylindric shape,
they could be installed from the outside with the cam in place. The later
version has tee shaped tappets, they have to be installed from the inside of
the cam housing, before the cam shaft is mounted.

There is nothing subtle about the tappet shape, if you have one out you
can't miss the larger diameter at the cam shaft end. The cylindric tappets
often get a very small ridge turned up around the rubbing face, this should
be stoned off, then the tasppet will insert easily from the outside.

If your cam housing is like the one on the left you have cylindric tappets,
and you can use the timing pin. If you have the extended tappet guides seen
in the view to the right you almost definitely have the tee shaped tappets.
You don't have to pull the rubbers up to see this. The shape will be visible
through the rubber seal. Saito calls the version with the extended guides
the "High Cam" series.

References to "Left" and "Right" in the following notes are looking from the
rear of the engine with the carb and intake on top of the engine.

Setting the right cam is exactly like the single cylinder engines, all the
marks are the same. If you are using the timing pin stick it in the forward
(intake) tappet guide, and with the piston at TDC install the cam. Pull the
timing pin and stick the tappet in, you're finished with the right cam.

The left cam is ground in reverse, the front lobe is exhaust and the rear is
the intake. The two cams are have to be on the correct sides, if you switch
them side to side you have made the engine set up for counter rotation. Be
careful.

There's an unnecessarilky complicateed method in the manual for installing
the left cam, I think my way is easier. After mounting the right cam, with
the piston still at TDC and the right valves in the overlap (installed)
position, turn the crank one full turn bringing the pistons back to TDC, now
the right cylinder is a TDC between compression and power strokes, the left
cylinder is therefore st TDC between exhaust and intake strokes. Set the cam
in its housing to the overlap position, (you can feel the tappet movement to
find it) then being careful not to turn the cam set the cam and its housing
in position.

As memory serves the timing pin wont work on the left cam regardless of
which tappets you have.

For your final timing check install the rocker pedestals, push rods, and
rockers. Watching the rockers on the right cylinder you'll see the exhaust
valve open (turning the crank in forward rotation), as it is closing you
will see the intake start opening. When the two valves are equally open you
have reached the "Overlap" point I mentioned earlier. Use a thin probe in
the plug hole to feel the piston position, rock the crank back and forth.
From the overl;ap point any turn of the crank should make the piston go
down. If not, the cam is out of time, you're off by one tooth or more. Go
back and reset it, The left cylinder is checked in exactly the same manner,
remembering that the exhaust and inlet vaklves are reversed - the upper
valve is still the intake but it's backwards in relation to the left side.

When you're sure the cams are timed correctly go back and do the final
assembly of the valve gear. Set all the clearances as close to 0.002" as you
can. If you assemble the engine in the same order as I do this will be the
very last part except the intakie and exhaust manifolds and the carb. With
them mounted the engine will be ready to run.

There is a way to hold the cam, but it's a little awkward until you practice it. With the tappets in place set the push rods against them, then holding the cam housing with your thumb and third fingers press the oush rods in with your index and middle fingers. You can then reach under with your other hand and turn the gear, you'll be able to feel when it's at the overlap point. And as you insert the assembly you'll feel if you have bumped the gear and knocked the cam out of position. This works on both sides. And after a little practice it works much better than grease.

OIne thing I forgot to mention is the gasket for the cam housing. You can use the stock gasket with no problem, but if you try to assemble the engine with no gasket the timing gears will bind. The gear lash is adjusted with the variation of the gasket thickness, a high time engine often will be quieter if the lash is reset to a tighter point. The clicking you hear as you turn the engine by hand is the lash in these gears, by setting it tighter some of the noise will go away.

It's rare for these engines to have an internal failure except at very high time. What let go on yours? Oh, don't forget the teflon pads on the wrist pins as you put the pistons in.