I think a standard Bosch style relay, which are generally available in 40A capacity would be fine to use to power your switched fuse block. Unless you're putting some crazy loads on that fuse block, which it doesn't sound like you're doing. If you need additional relays, the ones that dovetail together are nice. I used 4 of them to run my turn signals and mounted them to the steering column clamp support.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.