Do you still have the distributor apart? In a Chrysler built distributor there is s number stamped near the advance slots (underneath). Multiply by two will be crankshaft degrees. Look at the slots and seee if someone welded or brazed up the slots.

With the distributor apart, but springs and weights installed, push on the wieght with the light spring as if its was moving out with centrificgal force. It should rotate the slot armature unil the loop of the long spring has no slack. There should still be a 1/3 to just a little bit of the slot left at that point. On some mopar high porformance distributors the heavy spring lets the advance continue very slowly preventing bounce and offseting any retard caused by stretching and twisting components. This is how the mechanical tach drive distributors were made. On a more run of the mill distributor, the heavy spring just shaped the advance curve.

Based on the specs you've given, everything else seems pretty good. I ran 340 with comcams HE280, 9 something compression and singleplane street dominator, and yes a 3310 on top. Now I'm running a similar engine with the LD340. 18 degrees is a good ballpark initial and 34 or so is a good initial target for 2800+ rpm. One thing I didnt notice was if you had a 4 speed or 727. I used a torque converter with 3000 stall. I've had that reduced to 2700 or so and its not as good - much harder to dial in the timing and carb especially with the 10% ethenal mixed gasoline. I'd say a high stall TC is required for your combo just as it was for mine (or higher gears, or , or or )

Cheap way is find used (in good shape) or parts store rebuild. Or if yours is in good shape then just finda few others so you can take parts out (and get the slots you want). The MP distributors used to be a good bang for the buck but not sure how that will work out with the Pertronix.