You might find a cartridge bottom bracket that will work with your threading. I have never personally done this but you can find someone else on this site that has more expertise with this easily. I switched my Grand Prix over to square-taper by swapping out the spindle only. The hardest part of this is finding a spindle that will give you the correct chain line. Check on Sheldon Brown's site to find how to measure this and estimate which new spindle you can use. Any of the Japanese spindles should fit into your bottom bracket, it's just a matter of finding which one puts your cranks the right distance out from the frame. I ended up using a 3P spindle, but something just a hair longer would be better. You might find one of these on eBay or in an odd corner of the internet, but I dug through the parts bin of my local co-op until I found a spindle that I was satasfied with. You will of course need a new crank set as well, but these can usually be found on eBay, especially if you're not too picky.

As for brakes, I found a pair of Weinnman center-pulls at my co-op. I like center-pulls much more than side-pulls, especially on vintage bikes, but the side-pulls will also work with this frame. There are possibly going to be some issues with reach, especially in the back, but you can measure the distance from the brake bolt to the rim of your tire to figure out how long your brakes need to be. You might take the opportunity, if you're going to switch out the brakes anyways, to add an extra 4 mm of reach so you can swith to 700c wheels in the future.