Just to throw an alternative POV in the mix regarding the age old 1500 vs 1850 debate.....
There's not a lot between an 1850 manual and a 1500 od MPG wise, a well sorted one of either will be in the 35 - 40 range.
I doubt there's a great deal between them off the line either, 1500 mechanicals are enormously lighter. Up to 60 at any rate, the 1850 will have the edge once a dolomites marvellous aerodynamics take over.
I don't think there's much between them on the maintenance front either, both need the bonnet up every 3k miles plus the annual summer lube/tighten/adjust session.
Both are twin carbs which need looking after. Dissy on a 1500 is much easier, no tappets to arse with on an 1850. 1500 engine bearings need changing as a 20k service item, 1850 is closer to the torque capacity of it's gearbox & axle.
There is nothing at all between them handling wise, they are the same car. The weight I think is perfectly balanced in a 1500 and I'm inclined to jump to 'it can't be better in an 1850' but I've not driven an 1850 enthusiastically. Honestly, they're probably so close it's down to the tyre choice taking the edge.
Advantages of an 1850:
No really, much MUCH more like a modern than a 1500.
Advantages of a 1500 od:
If you have the tiniest bit of mechanical sympathy you can't kill one. Throw as much abuse as you like at an all iron triumph. Overheat it, ignore <insert any maintenance procedure you like here> and it'll keep going until it explodes. Have that tiniest bit of mechanical sympathy and it'll get you home. There's very little indeed on a 1500 you can't ignore until you simply cannot stand the noise any longer. When you get home it will be cheap to fix every single time other than the od box (same box on an 1850 by the way). Overheat a 1850 just a little bit and you're looking at a half walletectomy if you are lucky, the full YE_GODS_HOW_MUCH if the head studs do their much fabled trick.
So if you want easier than a mini to look after and indestructible at the cost of spinny and unrewarding performance wise then 1500 od.
If you want more refined than a mini, a cruiser, are prepared for more complex mechanicals and can cope if the fit hits the shan then 1850.
76 Mini 1000 auto, about 30k miles experience in and looking after for Mrs.
80 1500 later converted to od, north of 105k miles every one under my foot.
Helped fix dozens of 1850s and travelled in a few.
Sprint owner and perpetual fixer for far, far too many years.
Edited to add another thought as to how you might look at it:
When I was commuting once a fortnight from near Gatwick to Manchester my 1500 was ok. Only OK. It needed the overdrive I hadn't got at the time. I only did about 6 months of that.
When I was commuting daily from Gatwick to Heathrow (35 each way) and later Bracknell (40 each way) my 1500 with od was exemplary. Holidays in Scotland, Cornwall, North wales, Cumbria all no problem at all.
When I was commuting weekly from the Forest of Dean to Horsham (150 miles) and daily in between about 10 miles I really needed the longer, more relaxed legs of the 1850.
There's not a lot of difference in the latter two weekly mileages but long, long Monday and Friday week after week did not suit the 1500 even with od.
1978 Pageant Sprint
- the rustomite, 1972 Spitfire IV
- sprintfire project, 1968 Valencia GT6 II
- little Blue, 1980 Vermillion 1500HL
- resting. 1974 Sienna 1500TC
, Mrs Weevils big brown.