Since an RX-7 is rear-wheel drive, I'm thinking you'll have to find some way to disconnect the driveline. Remco actually doesn't list any products for it and denotes it as "Must be towed using a trailer."
You can check their "Tow My Vehicle" page to confirm ...click here
The "general" rule used to be that any front
-wheel drive vehicle with a manual transmission can be flat towed ...again, it was a general rule.
ETA: ya know, I may be entirely clueless as I just did a quick search and found this thread on an RX-7 enthusiasts forum. One guy said he did tow his at freeway speeds without disconnecting the driveshaft ...I don't know. Maybe start a thread over there to ask the question?
Here's the thread I'm referring to
However, this was the exception as most of the search turned up answers that said it can't be towed 4-down.