I have a rover v8 in mine, its a shame photobucket has to be such a d1ck otherwise I've loads of pics to share. Rovers are still plentiful and cheap as is to buy, however I understand both the SBC and SBF are a fraction smaller. It will be just as much effort to put any v8 in but something worth thinking about. 302s and 350 chevs cost a bomb in the country though. But again if you want to modify any of those engines for power it will cost the same and you'll get more from a SBC or SBF.
My rv8 is in a mk3 bay with the least amount of mods to the shell - only the inner valance and radiator mounts have been removed to fit the radiator. There are no mods to the bulkhead and gearbox tunnel, although lots of people do this for space so its up to you how and where you mount it and wheather you ever want to put the car back to standard.
Gearbox wise, if you arent bothered about cutting the shell, a Lt77 or later rover 5 speed box will go in but because it is a tall box you need to cut and raise the gearbox tunnel by an inch or two. When i had a 3.5 rv8 in another mk3, it was mated to a type 5 4speed (3.0 capri) gearbox using a rover auto bellhousing with a plate on the back of it that had the holes drilled to fit the ford box. That box fits in the tunnel fine, however you have to enlarge the hole where the gearstick pops out as it all sits further back. Its a strong enough box for the 3.5 as that engine only makes about 130/150bhp. But then I upgraded the engine to a 4.6 Rv8 and the box wasnt happy. That box has been rebuild twice but I am upgrading to a Borg Warner T5 5 speed. My old box and bellhousing very well might be for sale soon.
The T5 I have is from a sierra cosworth, however in the TVR Griffith and Chimeara the T5 was behind the high powered rover v8s so you can buy a bellhousing built for the job although its been my experience they are hard to find! And as Rich says, they are set up for a hydraulic clutch set up. I have just bought another conversion bellhousing for it using a rover auto bellhousing and a plate bolted to the back - The reason the auto bellhousing is used is because it has a flat face where it bolts to the box so its easy to use a flat plate for whatever gearbox you want. I have one of these for sale at the mo.
There are lots of variations of the t5 with the main differences being the gear ratios and tailhousings where the shifter is mounted. I have gone with a cossie box as it has the best gear ratios for my purpose and a standard ford type 9 etc main shaft so I can retain the same clutch, spigot ect. I have gone for a 80s/90s mustang tailhousing as this puts the shifter in the right position and also is one of the only other versions of the box with a mechanical speedo drive. The TVR T5s and later Mustang T5s use electric speedos. Although I still need to get my prop shaft yoke moddified and the speedo drive repositioned on the main shaft so its no walk in the park. But essentially the T5 does fit in the tunnel! They are expesive though!
I also have a mk3 capri with a 3.5 rv8 mated to a Sd1 auto box (Borg warner BW65) which works very well and i am pretty sure would fit a cortina tunnel without mods but dont quote me on that. Again it depends on what you want from the car, a lot of people turn their noses up at auto boxes but having one behind a lazy v8 ..... happy days! Any more questions please feel free to ask!