I'm working on the Merc with a 312 and 3 speed automatic (P R N D L). The owner says he can outrun this car for the first 100 yds. We checked the timing, 6 BTDC. We checked the compression. Ranged from 125 to 150 about a 50/50 split, so not at the best. He uses leaded gas, not sure about the octane. We adjusted the single 4 bbl carb. This engine fires right up, runs smoothly, sounds great. Shift into gear and the torque converter engages solidly (bumps the car forward if the rpms are a little high). This car is stock, other than the air filter. The owner recalls from youth that these cars had more guts that what he's seeing.

So any suggestions on boosting power? 

He also mentioned that he only feels the car shift once. I know it's a 3 speed, but why only one shift (not sure this is related to the lack of power). Starting in 2nd? Not sure that's possible. Any linkages adjustments that might be off? Thanks.

You have a timing light. Rev the engine and watch the timing mark on the engine. If it does not move your advance vacuum can is not working making your engine have next to no power.
To check either or both vacuum cans remove the distributor cap, put a length of hose on each line one at a time and suck on it. The plate containing the points should move.
The dual can is expensive to buy but since you have one it can be rebuilt for a lot less money. The Carter WCFB was available from one of the assembly plants and for most a more desirable carb that still needed the loadmatic distributor.
Most here have upgraded their carburetor and installed a 1957-1962 distributor. It was the first thing I did to mine. Good luck.

56 Vic, B'Ville 200 MPH Club Member, So Cal.