I've not seen a 28/36 manifold but that doesn't mean there isn't one out there somewhere! Or an adaptor to go with a single carb Toledo or 1500FWD manifold. But I have seen, quite recently, a manifold to take a single 40DCOE/DHLA, I think Jod Clark or Richard Brigstock had it for sale.
With regard to final drives, the lowest practically available one is the 4.11 fitted to Heralds/Spitfires1-3/Toledo/Dolomite 1300. There is (or was) a lower one at 4.875 (IIRC) that was used on 948cc Heralds but a) very rare, b) made of cheese, c) not sure the half shaft spline is the same as on a Dolly axle. I hear rumours of a 4.11 LSD, which would be the holy grail of diffs for such a beast, but I think this is borrowed from the Compo Triumph 2000/TR cars,(if it does exist) so would need to be in a Sprint case.
The ordinary 4.11 is not terribly strong either being only designed to cope with 60odd bhp, but it will go straight into a 1500 case and second hand ones are plentiful and quite cheap (which is handy if you are going to break a few)
Other choices are 3.89 (early 1500/1850 Dolly/SpitIV1300/Vitesse 2000/GT6 with O/D)
3.63 (late 1500/1850 Dolly/Spit1500)
3.45 (Sprint, has to have a Sprint case, I believe a 3.7 from a T2000 can also fit this case)
3.27 (Dolly 1850 auto/GT6 non O/D)
Of these, the toughest one to use in the 1500 case is the 3.63 which is not very low but can be built with uprated bearings and will cope with up to about 120bhp.
If I was doing this, I would be inclined towards the Sprint axle, which is strong enough to take the punishment and get the gearing advantage with an overdrive gearbox modded to give overdrive in 2nd gear too. Maybe try for the 3.7 conversion but this would need to be custom built into a Sprint case as it's not an "off the shelf" choice.
PS, there is not a lot of choice in the gearbox department, though there are several boxes that can be MADE to fit from the Triumph range, they are all much of a muchness as far as intermediate ratios go. Or If your regs permit, you could consider a Ford type 9 conversion.
2 door '73 Toledo with Vauxhall Carlton engine OWF 797M (The Carledo)
Vermillion (and Rust) Sprint Auto EGP 247T (The Dolomega)
'91 Cavalier 2ltr 8v auto
'95 Cavalier 2ltr 16v auto
Spectrum Auto Services, Servicing, Repairs, MOT prep. Apprentice served Triumph Specialist for 40 years and home of Maverick Triumph.PM for more info or quotes.