The thing that normally stop people in their track when it comes to making a given 5 speed tranny workm in a given car, say like a MGB is the custom mmachine work involved. The Rivergate Datsun 280Z MGB 5 speed conversion uses a speical custom made engine back plate, a modified early MGB flywheel to put a Datsun starter, a aftermarket slave cylinder, a custom made clutch disc, and a custom made pilot bushing. David Headley does a 5 speed kit for the MGB using the Borg Warner T5 using Mustang and Camaro bellhousing, a speical made rear engine palte, a custom clutch disc, and some itme and need the chassis relieved to make room for the large gear box. The T9 I sell and other is really nice kit and includes a gaurateed gearbox, and has special cast bellhousing that is a exact copy of the front bellhousing section of original MG gearbox, so on a MGB the T9 kit uses all stock MGB clutch parts. and of course all these unit require a custom driveshaft.
Ok that being said, if the transmssion you choose has a intergrated bellhousing in t design like a MGB gearbox , it needes to fit the chassis and, have a special custom made engine back palte, then it come down to input sahft length and diamet wroking with the given engine you are working with, if it require you modify the input sahft, then you already are headiong in the worng direction. if the gear box has a bolt on bellhousing like the T5 or T9, and the sotck one for that doesn not work, you going to have to make a bellhousing, this is not a project for the weak of heart, I done this before on Midget engine to make a custom bellhousing to work on engine dyno with a MG midget A series engine, it was alot of work, I know Bob White has done the same thing with a T5 kit he made for himself on a MGB using a MGB bellhousing section off the MGB gearbox, same sort of thing i did with the Midget.
Long story short, nothing just bolts up, and if the input shaft length and diameter is not right, then it will not work without internal transmission mods, meaning it can't be reproduced for others easily. So you're going to have to a be cracker jack tool and die machinist or be real good friend with one, or you'll probalby spend as much money to get a new kit going as buying one the already existing ones. Moast peopel that have tried this that had to rely on paying a tool and die machinist have said they spent $2500 or so, which gets you real close the best engineered kit out there, the T9 kits,and of course if you're basing this on a junkyard gearbox, that can always be a roll of the dice as well.
In closing, it's not a project that can't be done, thats how all these kits were developed in the first place, but very specialized machining will be involved as well as custom made parts, and low volume design machine work is not cheap, The guys who made the kits for the masses had to sell quite a few of them to ever see a profit after their initial investment. Of course some people design these kit better than others, and IMHO Peter Gambrell who designand had all the piece made up for the T9 kits was a very samrt cookie, he designed the kit ot be simple and rely on stock parts that would be easy form the person to get down the line, where some of the kits use special made parts that the supply will dry up for over time, nothing would be more depressing to find out 3 years after you got a oddblal 5 speed kit is now you can't buy the custom made clutrch disc for,and now you have to paly engineer to just reclutuch your car. Sometime it is just best to just pay the money, get the good stuff, and move on, rather than reinvnet the wheel, unless of course you are tool and die machinsit with full machine shop at your disposal and really good at design, which some of the folks that made certain kit were not.
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