chevy crate motor timing trouble

[brian]

I put a new GM crate motor in my car. It's 300HP 350 and I used a Mallory
Unilite distributor that does not have a vacuum advance. It has a 6 3/4
balancer so I got the timing tabs for it and bolted them onto the timing
cover. I fired the engine up and did the break in but after the break in I
went to time the engine, 10 degrees before top dead center is what is
called for by GM, and when I turned the distributor to retard the timing
back down to 10BTDC it started to run extremely bad and strarted to
backfire. If I were to guess, by just eyeing the position of the timing
marks, it runs good at maybe 20 degrees or so BTDC. Do you have to account
for the fact that it does not have a vacuum advance in the initial timing?
Are the timing tabs I got wrong? It said they were for the 6 3/4 inch
balancers. But if I try to put it at 10 BTDC it will not run. Could I
have put the distributor in wrong?

[DaddyMonkey]

did you put the "timing-tabs" on correctly?

are you sure?
are you positive?
are you sure you're positive?

put a small wooden dowel through the spark-plug holes... carefully turn
the engine over by hand. Note the placement of TDC at the piston travel
to the timing marks on the "timing tabs"

[Edward]

Make sure idle speed is low enough so no ignition advance takes place
in the distributor. Check this with a timing light as you change
engine speed.

Turn distributor apposite direction of rotation of rotor for advance
of spark and see if strip with degrees on it shows advance, if not,
reverse strip.

Multiply diameter, 6.75 inches, by pi, 3.14, and divide by 360 degrees
to get inches per degree, which works out to about 0.6 inches for ten
degrees. Take a tape and see if your strip gives this value for a ten
degree change.