Since you have an MSD adjustable timing box set the timing to 25* initial timing with VA disconnected. +-7* adjustment will allow you to figure out the best idle timing. Run the engine up to full operating temp with the hood closed and turn the engine off. Wait 5 minutes and allow the engine to heat soak. Adjust timing knob to the point the starter does not kickback and the engine instantly starts. Check timing with a timing light, this is the initial timing your engine wants. Set up the distributor for max mechanical timing 34* is all I've ever needed on my 340. As an example my 340 10.5:1 with
[email protected] SFT cam has run for years at 28*/34*/48*with ported VA starting to move at 8" .
VA also allows you to add timing when the engine is under light load, low throttle opening conditions (cruising). It also keeps the engine coolant temps lower and improves mpg.