Without being too sure of how much current has to pass through the regulator, an alternative to paralleled transistors (pcb space) is a high current linear regulator, or better yet an LDO, you can get 5A ones
e.g.
LM138/LM338 5-Amp Adjustable Regulators
LM1084 5A Low Dropout Positive Regulators
MIC29150/29300/29500/29750 High-Current Low-Dropout Regulators 1.5A to 7.5A
There are a lot more to choose from. Not sure if a switching regulator is adequate for that part of your circuit instead.