Oh my goodness...Hercules flat head 6's...We had two (MRL and MRLO for 12 yeaRS,)
To the best of my memory, no vacuum advance. Springs ONLY control the ignition advance and it does sound like your problem is ignition.
Try to find an OLD auto shop with distributor set-up equipment. They should be able to set advance angles (based on weight throw out data) for your distributor.
I refer you to... http://www.classicboatwork.com/manua...20ML%20MBL.pdf
for for carb' adjustment (if mod's allow).
Pretty simple and robust engines.
Hope this helps...