Just so we aren't mixing terms, curb weight is defiined as the weight of the car, with a full tank of gas, but no cargo or occupants,
GVW (gross vehicle weight) is defined as the maximum rated weight of the vehicle, including car, occupants, fuel, and cargo.
If the car's curb weight is 3100, you can probably expect the "glider" (car minus engine, gas tank, exhaust, etc) to weigh in around 2300-2500. This means to stay under GVW you have about 1600lbs to play with. EV components (everything but the batteries) will probably add up to 300 to 400lbs or so.
For an EV, as long as you upgrade brakes and springs appropriately, I would not be too terribly worried about exceeding GVW as long as you don't go crazy. Depending on the car you might be able to bolt on bigger brakes and heavier suspension from higher performance/bigger engine models. I don't know enough about volvos to be able to speak to your specific situation though. While it is possible for me to cram 3 more T-105s in my car, as far as I am concerned it is at its maximum safe weight, especially the rear end (I have 7 up front, and 14 in the back).
For a regular 42 mile freeway round trip in a 245 wagon, I really would shoot for 24 6v batteries. a pack of 24 T-105s would weigh about 1440 lbs, and a pack of 24 T-145s would weigh around 1730lbs. All else equal, range would be proportionally better with the T-145s, and acceleration would be better with the T-105s owing to lighter weight but same voltage.
Batteries have a finite lifespan whether being used or being stored, so batteries should be nearly the LAST thing to be purchased for a conversion. You can use a few old batteries, or buy or make mockups to use for fit and sizing purposes during the build. Yes prices are going up, but it will cost a lot more to buy TWO packs, because the first one sat around for too long.
The other thing you should do (which I did not and am now kicking myself for) is ensure your battery boxes are insulated and can be heated during winter months, otherwise your range will suffer during the winter. You are pushing the limit on practical daily range given the size of the car you want to convert so it would probably be fine during summer months but without heating you might have trouble doing 40 miles during the winter.
If you can find a smaller volvo (maybe an older, classic one) you could build a car that would do better in terms of both range and performance than the 245 owing to its smaller size and lighter weight. If you don't need/want a 245 wagon specifically, then I strongly recommend investigating your options. The chassis is a big determining factor in the final range/performance of an EV so it makes sense to pick the lightest, sleekest one you can. As for fixing things, you will find on an EV that aside from brakes, you are doing more the part of an electrician than a mechanic most of the time anyway so I would not be too worried about your familiarity with the car. I don't know a darn thing about toyotas, and that sure didn't stop me...