Another mid-1960s 3-400 amplifier meant to use with radios that had tube-type final stages. The 3-500 replaced the -400 in 1970, pretty sure. It's taller, so you have to drop the tube sockets a bit to provide overhead clearance.
The complete lack of a tuned input means that the Load and Tune controls on the radio will peak differently driving the amplifier than driving an antenna. There are other drawbacks to this "untuned input" design, but we'll skip those for now. A modern solid-state HF transceiver will reduce power or shut down altogether trying to drive this amplifier.
If this was the radio equivalent of a Jaguar "E" or a late 60's 'Vette you could make a case for updating it to use again. But the Tempo 2000 is more like a 4-cylinder short bed pickup from 1965.
The Tempo won't have a rusted-out bed, but being sure the big, expensive transformer is okay would be wise just the same.
I ended up the accidental owner of one of these. Never have talked myself into rehabbing it.
73