Leg strength imbalance on bike

I was given a Garmin Vector S single pedal power system for an early Christmas present last year and was excited to be able to train with power on the bike.  After what was a really easy installation, I was a bit disappointed in my power numbers.  My previous exposure to power was on Trainer Road and I  had an FTP of 230W and with the Vector S I could only manage to get a 190W FTP during the test.  I put that off to being at a different point in my training, coming back after a long left leg hamstring rehab, and perhaps a bit of variation between the Vector and TR (TR claims it can be within a few percent of power meters). 

I then read a number of posts about single pedal systems and how they could potentially be showing an inaccurate power value due to leg strength imbalances.  Since it measures only the left leg and doubles that value, if the left leg is significantly weaker than the right leg, the power value on your computer is necessarily going to be low.  That got me thinking and wanting to upgrade to the dual pedal configuration.  I pulled the trigger and got the upgrade, installed it and went out for a few rides.  Guess what, power was up.  I was sustaining 180W for 90 minute rides at a comfortable/hard effort (HR level 1.7 - 2.4 depending on hill content) and found my left leg is definitely weaker than the right. Based on an albeit small sample size of rides, some flat and some with rollers and small hills, the numbers consistently come in at 43% left and 57% right.  It is not a big surprise that there's a difference and that the left leg is weaker since that is the leg that had the hamstring issue, and now I have to address it.  I have not done an FTP test since I got the dual system and I wanted to see what it would show me, but will be doing that in the next few weeks.

I'm looking for guidance on strengthening exercises and on-bike drills/guidance that will allow me to bring the R/L leg strengths to an even level.  I've got access to a gym with all the free weights and weight machines and have a trainer I can put the bike on to safely focus on the leg without having to worry about surrounding conditions.  Current training is still in base building for next season with my A race (Vineman Ironman) being at the end of July.