According to Landis & Jenkins' huge book Valkyrie: North American's Mach 3 Superbomber, most of the studies focused on IR signature reduction, and some of those got pretty detailed. They do mention that RAM applications were studied, including on the inner surfaces of the intakes. Remak and Ventolo's book XB-70 Valkyrie: The Ride to Valhalla also mentions the RAM treatments (although in far less detail), and they've got a polar RCS plot showing the RCS of the XB-70 from various angles. From head on it appears to have a very small RCS for its size of well under 100 m^2, with a 1,000 m^2 spike to the left and right of center due to the untreated intakes. RAM would've cut those down significantly giving the XB-70 an RCS of well under 100 m^2 to an angle of about 45 degrees left and right of center in the forward hemisphere. From the side, yeah, it looked like a small moon on radar! But so did any other large aircraft at the time, really.
So, while side-on both of them were massive targets, from directly forward the B-70 design did have a greatly reduced RCS compared to the B-52, which is often quoted as having an RCS of 100 m^2. It wasn't anywhere near being stealthy, but it was a huge improvement for the time, and would've been further improved through the addition of RAM.
At the very least, B-70s bombing Vietnam would have been far more survivable than the B-52s employed. OXCARTs and Blackbirds proved that Mach 3 at altitude could beat the Vietnamese SAM systems.
