I've overhauled a couple of 1098's and found them to be a very simple engine the 1275's were the same as I under stand it. No oil to the rockers (vapor) and no rear main seal.(labirinth) Mine were in a 65 Sprite with SU's. For sizing I'm sure the BMC will fit easily not sure with the Datsun. A rear from a Sprite/Midget is narrow and mounted on leafs. They had 4.22 gears and we also found some 3.70's. My normal fuel milage was over 40 on the road and 35 in town if this helps.
It would be really cool to get on of these real modern computor controled DOHC, convert it rear wheel drive and use all the controls. Good Luck
