I don't think getting the USA to ally with the soviets is possible, unless you coup them or sth.
nuclear tech: cut and paste the nuclear technologies from the "ignore" list to the endgoal and prefered lists.
Building reactors: check the USA AI in the last DAIM release. The US is set to possess nuclear weapons by 1945, so just check their build priorities / script.
Just remember: Having nukes is not enough. You also need a delivery system (strategic bombers or -after researching miniturized fusion- rockets). As it is now, the US (and IIRC) USSR are scripted to "avoid" the relative techs, so you will also have to change that.
Personally, i believe that the soviet AI shouldn't be bothered with nuclear techs up until 1944-1945. Using research plots and spending huge amounts of IC to build reactors should wait until after german defeat (as in RL).