From Street Modified Classification - 16.1 Part G:
"Subframe connectors are allowed, but each connector must be attached individually without any lateral components attaching
the two longitudinal frame rails. Subframe connectors may be bolted or welded. SP allowances which authorize unrestricted
mounting of suspension components do not permit the incorporation of non-SP-legal subframe connectors into the
mounting scheme for a Street Modified vehicle."
Since the H-brace has lateral components attaching the the two longitundinal frame rails it is not SM legal. If you are really serious about under-body bracing(with regards to SM) look into the Cusco v1 subframe connector as it mounts from one frame rail to the other without any additional bracing. it is very simmilair to a tie-rod connector
For Street Prepared classes, BMW guys are allowed to use subframe connectors because convertible models came with them from the factory(up-date-back-date rule)
What happened to the BMW you were looking at in this thread:
http://www.unysoc.org/board/index.php/topic,2450.0.htmlOr like beat22 said, get a miata
the 3 series or miata are wonderful chassis for building extremely competitive AX cars. With minimal or no mods, bmw's or miata's are simply teh fa4st0r