As others have said, batteries do get lazy in the very cold weather we have been having and you should, as said, check all the electrical contacts and earths to and from the battery, the engine earth strap where it bolts onto the block and cleanliness of terminals on the starter motor.
Also check that the small headed bolts on the back of the starter are tight, I've seen many pinto's over the years where these have loosened causing very lazy starter motors.
The ignition timing needs strobing up to about 6 - 8 degrees BTDC for best chance starting. (Time with distributor vacuum pipe off and plugged with a screw )
I trust your plugs, leads and distributor cap are OK? Other things to think about is how old is your engine oil? Gloopy old oil hinders cranking and if it's not been changed I'd recommend trying that and using some 15/w40 during the cold weather.