I just set my timing on the 75 -350ci Using highest vac. at idle. Now it feels like a bad miss in the engine or has a bigger cam. I don't know, it did ping a little under load so I st it back a little. Any ideas?

The very best thing to do is go to AutoZone, Advanced, NAPA, Summit, etc, and get the timing indicator and put it on your engine. Then and only then will you have some idea what you are doing.

The vacuum method can somewhat work, but can also let you down depending on your engine and equipment. If you don't want to get the pointer take out Number one spark plug. Disconnect the power to the distributor. Now use something soft but sort of stiff like a soda straw or zip tie. DON'T use anything hard that can damage the cylinder or plug hole, or get broken off inside. Insert it in the spark plug hole and turn the engine by hand until the pistion is at the top. You may need to turn the engine back and forth a bit to get a good feel. When you are sure its at the top, use something and put a mark on the engine next the the timing mark not the balance wheel (pully support).

It doesn't matter if you are on the compression or exhaust stroke. The timing mark is the same either way. Which stroke is a function of the cam.
Reconnect distributor power.

Now you have something close to top dead center. Now use a protractor like in math class, and find however many degrees you want to set the timing at, go counterclockwise from your first mark that many degrees, and make another mark. That's where you will set the timing. If you were VERY carefull you will be very close, if not right on. Two degrees can make a noticable difference in engine performance and mileage.

I you don't have a timing light with the engine set on your new mark, take off the distributor cap. If you have HEI rotate the distrubutor and line up the teeth or points on the distributor pickup with the ones on the shaft. If you have points turn on the key and connect a test light to the negative side of the ignition coil. Slightly rotate the distributor until the points just open, and the test light comes on.
This is called static timing. Again, if you are carefull you can be within a few degrees, or right on.

Other than that it's just turning the distributor until you get lucky and are happy with it. Turn in very small increments.

Good luck.

did you unhook all of your other vacuum from the intake..

the vac gauge should be the only thing hooked up to the intake.

also you must make sure you unhook and plug the vacuum advance hose from carb..

this will give you erroneous timing regardless if whether you use a light or use vacuum.

Thanks guys, 1st I just ordered a timing pointer from NAPA, 7" balancer or 8" mine balancer is like 6 1/2" they said the 7" will work fine. About the intake vac lines, all that I unhooked was the vac.advance on the dist. This could be the problem. Also I ordered new plug wires Thinking I have a miss and it could be the plug wires. They look like a real pain to install in a stock L-48 with the ram horns still on.

if youve got a miss.. check for a fouled plug...
much more likely than a bad plug wire..