An Intellicon won't buy you any condensing efficiency the way a Munchkin Contender would, but a 6x oversized mod-con would be a bit silly, and would likely need a buffer tank to keep from short cycling (unless you're a hydronic-design genius and can set up the place perfectly as a single zone.) A right-sized Contender is around two grand new, with warranty, etc.

Installing & setting up a mod con is more than a simple plumbing project, and not a suitable DIY project. (Just buying the necessary tools & training to do it right would be expensive.) If you're undaunted by that, I STRONGLY advise you to seek out free/cheap training sessions sometimes offered by mod-con manufacturers (usually through distributors), but they may require you to have a plumbing license w/ gasfitter certification to even attend. There are many issues that come up, eg, modulating burners are much more sensitive to gas pressure transients etc than old-school stuff, just for starters. If you have an 83% AFUE cast iron beast already in service that isn't on it's last legs, setting it up to run at best efficiency for the load at hand with an Intellicon is clearly the best financial decision. If gas prices quadruple you can revisit the mod-con idea, but the costs will be more than just the boiler price tag- there are many system design factors that need to be done in advance to get the most out of it.

Measure the burn lengths of the existing setup. If you re-configure to run it as a 2-zone system the burn lengths would remain the same, but the duty cycle would go up, increasing the as-installed AFUE (lower idle time==lower standby loss). Unless you've done the manual-J and measured the radiation size to verify the balance, odds are the two systems are NOT inherently balanced well enough to run it as a single zone. But with an economizer you'd cut the number of cycles roughly in half (about the same as running it as a single zone) lengthening the burns, yet shaving 10-15% off the overall burner on-time over the course of a day for a 10-15% fuel savings. (In grotesquely oversized systems the savings would be more.) Setting it up as a simple 2-zone system with an economizer boiler control IS a reasonable DIY project for somebody with good plumbing skills, and modest electrician skills, and wouldn't require much in the way of re-tuning the radiation size or re-plumbing the near-boiler stuff to guarantee you don't end up with condensing conditions in a non-condensing-tolerant boiler. (You'll get to experience the joys of purging the air out of the system once you have it buttoned back up too! :-) )

If the system(s) have a boiler bypass or other low return temp protection stratetegy, measure the temp of the return water under a variety of conditions. If it's under 130F for extended periods of time you may have to adjust it. (Often a boiler bypass with a thermostatic mixing valve will work well enough on a boiler that size, but for many a much cheaper ball-valve would be "good enough" if the cold start wasn't too cold for too long.) With your house tightening efforts the average and peak temps of your radiators has been dropping, which may be lowering the average temp of the return water to a relevant temp. For continuous operation 130F would be a hard lower-limit for a cast iron boiler. But running at the lowest allowable return water temp maxes out it's combustion efficiency. For every 10F drop in return water temp you yield about a 2-3 percent reduction in fuel use (a fact incorporated into economizer controls that allow you to program a min boiler temp to be met during calls for heat.) The radiation was probably designed to deliver sufficient heat in the house's pre-insulated pre-air-sealed state at 160F-180F water, but would now likely be able to deliver even design-day heat with 120-140F water.