I don't think that the RMS will allow a SBC into HX-WB, but things like extractors & Holleys etc. on any old 6/253/308 quite often 'go under radar' as far ADRs are concerned.

Many inspectors don't have the knowledge of the intricate differences between all the variations of ADR27/A/B etc. on all the different engine & transmission combinations. Also a lot of the necessary documentation is no longer easily accessible.

As long as the cars looks good & runs well & isn't blowing smoke or leaking oil & the engine is from approximately the correct era, it will pass.

Dr Terry