A-body axles are shorter from
the outside of the flange (where the drums mount)
to where the bearing seats on the axle.
Consequently; when the drum goes on, the shoes have to be in the right place. And the backing plate determines this. So you gotta treat the small-bolt pattern A-body stuff as a set.
If your axles are too short from the bearing seat to the splined ends, then they will not engage the side gears correctly and the factory center thrust spacer will not be able to take up the space, and it may be impossible to set the endplay.
If your axles are too long, they will go too deep into the differential case and possibly hit the crosspin. But now the thrust-spacer will be too long, and again, it may be impossible to set the endplay.
There is a generous amount of space inside the ends of the axle tubes to receive the bearings. But there is not a lot of adjustment on the end-play adjuster. And if you jack it around too far, the drums will rub on the backing plates or the shoes will rub on the inside of the drums.
The point is sometimes 28inches is not 28 inches.
Another point worth mentioning is the tires and wheels that you want to run play a big part in how narrow you build your rearend.
If you have a Demon/Duster, I think they have the same inner tubs that my 68 Barracuda has. If that's true, and I don't know that it is, then 295/50s fit right in there. I installed mine onto 15 x10 alloy wheels that had a 4.5 backspace, cuz I liked the deep-dish look of those. To make it work, I yanked the rear end right outta there, and measured between the backs of the rims standing centered in the tubs. That is the measurement I was looking for when the stuff came back.
I ordered wrong, and so;
I had to fabricate spacers to go between the bearing retaining plates and the backing plates, and I had to move the inner seals out a smidge. That got me into the ballpark, but now the axles were too short. Soooo, I welded a thin washer on the end of one, and badabing! I was in.
All this to install C-body axles with 4.5 bolt-circle drums, into an A-body banjo, using the A-body backing plates. cuz that is what I had.
The point is this: I see you are in the UK, and I have no idea what support there is for Mopars, so; I suggest you do this in two stages. I mean, that's what I would do.
Stage 1; grab the banjo you have, and mock it up with the axles, drums, and backing plates that you intend to run, and with NO center section installed; to determine the drum to drum distance. Then fabricate any old spacer to set the endplay to ZERO, with the shoes more or less centered in the drums from side to side, so that no rubbing occurs. Then, measure the amount of drum to drum distance that needs to be removed; as determined by the chosen wheels and tires, and then get the banjo narrowed to achieve that. Bear in mind that the perches may need to be moved. You cannot fit 295s in there without moving the springs inboard at least flush with the inner tub wall. AND, you have to leave room for the U-bolts to be oriented correctly. You can only narrow that banjo until you run outta U-bolt space; from center to center and from side to side.
Stage two; when you get your narrowed housing back, mock it up again, with an appropriate spacer. Then measure the spacer,compare it to the factory one,calculate the difference, then cut the axles equally to achieve that; and it is almost guaranteed to fit.
To finish it up, all I had to do was move the springs inboard,reset the pinion angle, and weld new perches back on..
EDIT:
oh sorry, I see you said 70Dart. You cannot install 295s in a Dart lol, so nothing changes, other than that. To install decent rubber, the springs still need to be moved.
EDIT 2:
I have 4.5 years experience with a 70 Swinger340/4spd/3.55s, from 1970 to 1975. I can tell you that My Dart was the most evil, unforgiving pos car, when running on the factory Polyglass E-70/14s. It was like driving on sand, always slipping and sliding. Even lane changes could be exciting. And if the back-end stepped out just a bit too far, she was guaranteed to spin out. I was so excited to get rid of those tires. ( I was only 17 in 1970, still in hi-school, and as you might imagine, buying tires was a major big expense for me. And that car burned thru 6 pairs of summer tires and 1 pair of winter tires including those stinking Polyglass tires, in the 4years I drove it.
I sure wish I wouldda had 295s back in the day. But the biggest I could fit in the Dart was G60/14s. Those are measured differently. IIRC a G was 8.25 tread width. In today's measuring system that might be about 250mm section width. And still waaaaaay not enough rubber.
The point is this; If you drive like me, you gotta put the biggest, stickiest, daymn tire in there that you can make fit. And If I was to hotrod a Dart I would do one of two things
1) NOT install anything bigger than a 318 4bbl, with maybe 3.23 gears, or
2) tub it, right off.
That car has waaaaaaaaaaaay too much mass behind the rear axle; it loves to pendulum, and to do 180*s. Get rid of the spare, and drive with the trunk empty.
Happy HotRodding.
