i used a tko 600 in mine, which i think is real similar to the t5 and I didnt have to cut the tunnel, although it is snug in there. You might look up the tko 500 and 600 and see how close they are to the t5 but I recall them being real similar.
Like CT said, real snug in there. If doing over, I would cut large section out of floor, add a flange to the cut out section and install w/ gasket and sheet metal screws. TC
I installed a 5-T in my '54 and didn't have to modify the tunnel but it did require a little frame surgery. HRP
I'm putting a t-5 in my car and I will for sure have to cut the trans tunnel. Its an automatic though! 8)