Marty Johns car is a manual tunnel (auto tunnel is alot bigger) in the coupe and it needed a small amount of massaging to get my skyline R32 GTS-T box in it. I would not go to the expense of cutting a tunnel out. The massaging needed were for small bumps etc in the gearbox.
It is tight but the best way to get an SR20 in a coupe (cause youre not allowed to modify or cut the radiator support panel in IPRA rules) is to drop the engine in the car with the crossmember out of the car and then attach the crossmember to the engine under the car and also the gearbox then bring it all up to its mounting position.
It would be much easier to be able to separate the radiator panel and slide it in that way. If you do that youd risk being penalised in IPRA, but in saying that they are struggling for numbers so I would hardly think that it would be an issue.

edit;;;oh the reason i went the R32GTS-T box was that its stronger than a S13 box and actually alot shorter. You will need to change the bellhousing to suit. (they are interchangeable) I drove Denniss' car at Bathurst with the S13 box and the gear selector I thought was too far back.