It is an exact (weight, dimensions, etc.) replacement (all except for the rust and prep work needed on a stock unit). With that said. Right after they were approved, a bunch of racers with "supposed' stock units complained at a very well known track (not going to mention it, to not embarrass the tech squad at that place), that the Johnson units weren't actually "stock" . Needless to say, the track bought all the templates from Johnson Chassis, and ended up bouncing a half-dozen or so of the "stock" units that had been at the track for years, because those didn't meet stock dimensions in the front clip/frame area. So, if someone at a track complains about the Johnson Chassis, it is probably because their metric chassis has probably been altered, and they don't want those templates anywhere near their stuff.
It is a great idea, and the stock units are going fast, if almost not gone.