Best place is to start with what you want as an overall width outside tire to outside tire. Then compare that to an existing set up to determine what you need. As an example I have a stock '34 axle, '46 spindles, F-100 brakes, and 5:60x15 bias tires on 5-1/2"Ford pick-up wheels. It measures 65" outside tire to outside tire. Dropped original axles are usually around 49", that would narrow it to 63-1/2". If you wanted it narrower still a Super Bell is 48", Chassis Eng is '47, and narrow Super Bell is 46". There is 4-1/2" inch adjustment in tread width just in axle selection. Tube axles do not look as good as beam axles; but if needed you can special order a Super Bell tube axle in a narrower width. Wheel and tire size can also be varied to a degree. Best to make some parts usage decisions on what you want the result to look like and not rule out doing it right because of the idea that some parts are too expensive. You surely don't want to widen the fenders, that always looks terrible.
back when full fendered "T"s were common, the "default axle was a 46" king pin to king pin any thing. Super Bell came up with their 46" tube axle Because of full fendered "T"s that Magoo's and a couple of other shops were building. You can buy a beam axle in 46" these days too, and that is where I would point you. But as Rich B above said, it really boils down to wheel and tire selection and back spaces. I would stongly debate a Super Bell tube as being ugly though... Back in the late forties the original Bell tube was considered the high twitch racing stuff to have.
You should probably measure the spring's width while you are thinking of changing things. I believe that the model T spring is narrower. If that is the case then you need a wider crossmember. Also your T's wishbones won't mount to a later axle. I have an original T front axle with the wishbones etc. And frankly those things are pretty spindly looking to me. Also the wood spoke wheels don't give me a lot of confidence either. Also consider what parts are going to be outside of the kingpin area. The T has little, but the later ones will depend on what you use. If I were in your shoes I'd look at a stock model A, and take some measurements. With the skinny tires it may be close enough width wise. Good Luck!
Absolutely to the above! The "T" spring is only 1 1/2" wide, where as a '28-'34 type is 1 3/4". You definitely want to do a front crossmember to pick up the later spring. I don't think I woukd run the 1 1/2" spring myself. You mentioned something about replacing the frame earlier. For the drivetrain you have, the stocker would function ok. If you do consider going farther with power later on, I would grab the Wescott's dimensions at the back of their catalog and make a new one out of 1 1/2" X 3" 1/8" wall tubing. Literally just an afternoons work, and it would be a bit more stable than what you have. Sure wouldn't want to see you have to pull the whole car down though....
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