I would rebuild them, ASP wholesale has the kits, it takes a bit of patience as you need to make sure to keep the isolated grounds in tact (isolating washers on all terminals). The problem with replacing them is the replacement will be an inferior chinese clone, they are no longer made, so for $120 you get something that isn't as good. I had 1 real Delco 27SI and a chinese clone when I bought mine, I got a second core 27SI from hillbilly wizard for $40 and rebuilt it to replace the clone, when you look at the details the clone is kind of shabby, the castings are thin, the isolating washers are thin, etc. If it is just the voltage regulator and that is all you want to change it is a $10 part at the back of the alt inside, it is what the exciter plug is hooked to inside. The only thing that takes some tools and finesse is changing the bearings, you need to make a support and have a press to press them out without cracking the housing and press them back. I use a cardboard tube that has 1/2" thick walls, it is what we get stretch wrap on at the warehouse. I cut it to the height I need and use it to brace the frame and I like that it has just a bit of give to cushion the housing frame. If money isn't a huge obstacle and being on the road is, I might get a spare core, rebuild it, swap it in, rebuild the other. Make sure they are both 27SI isolated ground though as a lot of people swap the driver side with a standard 27SI and that can be a disaster on the passenger side. I keep 2 isolated ground so in a pinch I could swap one over, I also keep the chinese jobby in my spare parts rebuilt for an emergency swap.
