On 8/8/2012, Phil Bates in Idaho Falls, Idaho wrote:
"Having just fit a MkII/Twin Cam gearbox in my 1500 MGA body - this is a real bitch of a job. The geometry is such on my car (1958 MGA) that there is no way to bolt up the backplate and transmission and slide the engine and transmission in. You must remove the transmission output flange, the engine harmonic balancer, and the timing cover at the very least. I also did some pounding and grinding".

Certain problems come with installing mismatched parts. Installing a Twin Cam type gearbox in a non Twin Cam MGA is pretty rare. That gearbox is fairly valuable to Twin Cam owners, so you can be money ahead by selling it and buying a cheaper original type gearbox for your car.

The first issue is that the 1500 prior to January 1959 production needs to have the stater relief bump added to the tunnel to accept the high starter gearbox. Second issue is that the MGA never had a harmonic balancer, and the original crankshaft pulley is much thinner. Third issue is that you might be going about it the hard way (installing engine and gearbox as one unit). It is of course perfectly acceptable to take such shortcuts and not remove the seats, carpet, floor and tunnel as specified in the Workshop Manual. But the extra bump on top of the Twin Cam type bellhousing is likely to have added interference when trying to install engine and gearbox as one unit.

If you install the gearbox first, including the rear mount bolt, then lift the gearbox to place the bellhousing against top of tunnel, the engine should slide in without removing the (original) crankshaft pulley (although the pulley touches the steering rack and may scrape the paint a little). I always do this with my MGA 1500, and the 1600 type should be no different when the rear flange is past the rear mount before the engine comes along. I have never removed the crank pulley to R&R 1500 or 1600 engines. Then again, I have never had to deal with a Twin Cam type gearbox, so I can't be sure until someone will report on trying it.

Difference then is that the Twin Cam gearbox has the rectangular bump on top of the bellhousing. Since this is a clutch arm pivot location (for the Magnette), it should be far enough back to not interfere with the tunnel when lifting the gearbox. If that bump does hit the tunnel before the front of the bellhousing, then you might need to remove the crank pulley, but I don't think so.

This has been a bit of contention on various BBS for a long time. A few people swear by R&R of engine and gearbox together. A larger number of people likely agree that it is easier to R&R the engine alone for engine or clutch service, and never remove the gearbox unless the gearbox needs service. My recommendation is to R&R engine and gearbox separately, mostly so you don't have to stand the thing on end to get it in or out). You don't have to touch anything inside the car if the gearbox does not come out. If the gearbox does have to come out, then only remove the center carpet section from tunnel, the tunnel top cover and the remote shift extension housing.

Having done this a lot, I can have the engine out in 70 minutes, and 20 minutes more for the gearbox. It commonly takes twice as long to put it back together with some cleaning, new gaskets and some adjustments along the way. I pulled engine and gearbox together once just to give it an honest try, but never again. Pulling both together requires tilting the assembly almost on end to get it in or out, and a significantly high lift height. It might also require removal of the 1600 type gearbox rear flange or the crankshaft pulley.