260bhp is certainly on (or just over) the limit of what the getrag 240 boxes can cope with, especially in a race car i would think as thats a more abusive enviroment than road use.
The 240 series is also used in some other German 80's/90's cars notably some BMW's, however it is a totally different outer casing. But i would think the internal gears should be the same, otherwise why give it the 240 name. So if you do try to rebuild one there may be a source of parts in german gearbox specialists, but there are none i know of in the uk.
Also the GT/e 240 box has the same internals as the 1800 box you use currently, but the outer casing is different. However you can actually swap the casings across (its not just a bellhousing its also the majority of the case, but the gear cluster is fixed to the tail section not the case) which is handy as the GT/e boxes get less abuse ususally as its a lower reving engine so there have typically been less rushed gear changes.
Another option that might be of interest is swapping the box for something else alltogether. There are now adaptor plates available for the XE engine to fit the Mazda RX8 gearboxes, either the 5 speed or 6 speed (6 speed has same top and bottom gears virtually but an extra in between for a close ratio box) both of which happily handle upwards of 400bhp without issue. The RX box is longer though and has the gearlever almost 4 inches further back than the manta box. They use a ford spline at front and rear so clutch and prop are not hard to sort. Just a thought that might be of use to you.