I had an old Neve 2254E in the shop to fix a faulty switch. It it was a 4PDT, tight quarters, vintage, expensive. The trick was in how to heatshrink it well when it was so tight.
I called my friend Baron who always has answers to these types of queries. He recommended using the ECG J-300 Micro-Jet Portable Butane Torch.
He also recommended using Alpha FIT-221 heat shrink.
The butane torch works great. I will be ordering some FIT-221 to check out. I currently use the 3M stuff.