Len,
I would leave the needle jets alone - playing around with the needle height will have the same effect. On my bikes, I have found that they run better with an 85 or 90 main jet and a slightly smaller cutaway on the slides - you can then compensate by lowering the needle.
If you decide to run on 'super unleaded' fuel, you may need to increase the jets even more as the 'super' fuel tends to have a higher viscosity (giving a weaker mixture). I continue to run my bikes on plain unleaded without any problems with valve seat erosion - I did try additives for a while, but found they made the petrol go stale within about a month.
According to the handbook, the standard settings for the 274 carbs were 80 main jet, a 4/5 slide, and a No.4 needle - that was for 1950's petrol, so some adjustment will be necessary for today's fuel (I'll refrain from calling it 'petrol'!)
Regards,
Eddie.
