Another consideration, if you're not all that familiar with his old amp... maybe actually put in some replacement bass amp speakers. With that open back, it would have sounded pretty anemic, but in the 50/60s, not much was known about cab design and porting. May it suited his volume and tonal needs perfectly.
BTW, I just sold my 65 Pro Reverb for $2900 ebay. I owned it since I was 16, and I don't miss it a bit; never could liked the tone of Fenders, even with Reverends and good tubes, too tinny and HF, not enough mids.