by 'Forcing' a pin hi (+5v) or low (0v), you are simulating what the 7474 pin would be doing to the transistor driver.
You need a small wire or clip-lead from 5v (or 0v) and attach it onto the IC socket pin where the 7474 used to be.
(the 7474 MUST be removed, while you're FORCING pins, or you risk damaging the 7474 output pins !!)
IF the transistor turns on and brings on the lamps, then you've proven everything from that pin/socket onwards is ok, and the problem is now the 7474 chip, or the signal that goes into the 7474.
Since you've replaced the Tranny, it's More than likely a shorted globe or connector issue, it pretty much always is....
MM.
