mines an omega box and fits quite nicely
the gear linkage needed to be modified quite a bit to fit in my tunnel but that was the easy bit as a suggestion i put my gearbox into the engine bay then slid it rearwards into the aperture as theres one bit you dont really need to modify then built the tunnel around the gearbox the rear section is in the standard book position its just the diagonals that are moved
the distance from the centre of the gear lever to the mounting holes on the box is 29cm roughly and my bellhousing encroaches into the engine bay 8.5 cm this rough position should get you somewhere near but be mindfull ive made a custom pedal box using the omega masters and the design fitted in but still is tight and you need to be weary of the point the hydraulic hose enters the box itself unless you plan to relocate it ?
personally the easiest way to progress would be build the rear section of the tunnel where its supposed to be ie sierra diff in std position but alter the sections that come off this to meet the other rails ie tt2 tt3 tt9 tt11 tt10 tt12 (guess who just picked up his book ) hope that helps a bit
x25xe !! thats a big old engine to squeeze in you sure it will fit? im having issues with steering colulm being close but imagine it might be tighter on yours !!