If you want to keep the down tube friction shifters I can't think of any reason to spring for new stuff or 10 speed. There are tons of great used 8 or 9 speed shifters out there that will work great for much less money. I just built an older touring bike with friction dt shifters and I went with a nos set of early 90s shimano xt shifters (triple) and they are amazing. Believe it or not the difference between 8 and 10 speed is very little especially if you are not using brifters on a fast club ride. Also, and importantly, consider the rear spacing. Spreading the rear more than 5mm can cause alignment problems at the dropouts and result in a broken or bent hub axle and even if you spread it to 135 from 126 successfully (for example) you still may not have enough space to use that 10th gear.

I suggest a clean set of vintage 8 speed derailleurs to go with the friction shifting. You will have a blast as well as a better riding and safer bike (not to mention aesthetics).