I have a diamond frame Bstone 400 which is just one step up from the 300. The frame as I found it, had down tube Suntour shifters which worked perfectly, but I changed them to suntour bar ends which work the same as the down tube shifters, but of course are on the bar ends.

I replaced the original stem and handle bar with Nitto dlx Technomic and a Noodle bar.

I found it with only one 27' wheel, and have put on mismatched 700c wheels(Super Champion front Ukai rear). 700c tires allow alittle more clearance for fenders with 700x32 Paselas. I'm thinking about putting 650Bs on it.

I'd stick with 126mm rear wheels as there are plenty around.

Some Bridgestones had cranks that were drilled for a third chainwheel. Mine was and I replaced the spindle and added a 28 inner ring. I also replaced the 52-42 outer and middle chainrings with 46-36 rings.

I put on a Brooks B-17.

If you go to the Classic and Vintage forum, you might find a lot of advice on Bridgestones.