paul,
I'm afraid these type of questions very rarely provide anywhere need enough information and if you were to get an answer it at best would be a best guess type! These questions really need to be put before an engineer with all the relevant specifications and material data and any standards involved.

Do you have an MTR for the 1045 & the A732?

                                            Strength Requirements
                              Ultimate Tensile (ksi)           Yield (ksi)                                      

A732 Castings, 1A, 2A, 3A     60, 65, 75 min     40, 45, & 48 min
                          2Q, 5N     85 min                60 & 55 min
              4A, 6N (.4%Mo)     90 min                50 & 60 min
                        3Q, 13Q    100 & 105 min       90 & 85 min
         11Q (1.75% Ni), 4Q    120 & 125 min       85 & 90 min
---------------------------------------------------------------

Is the 1.25" Diameter 1045 round bar cold drawn or hot rolled? Why ask? Because there is a slight difference in tensile and yield strengths with the cold drawn having tensile & yield strengths of 91 & 77 ksi, and the hot rolled having lower tensile & yield strengths of 82 & 45 ksi respectively... In short, you need to look at both MTR's (Material Test Reports) in order to find the exact grade of A732 and 1045 steel that you want to weld together in order to narrow down the strength of filler metal to use because of the wide variations in both members possible T&Y strengths according to each respective grade... Or you can call Lincoln Electric or any of the other major filler metal manufacturers and they should be able to give you a recommendation or two.

Respectfully,
Henry