I think that the reight answer is:

Why the Westies should declare war to the Gambinos ?

When the Jacobi center must be build even the site was in Hell's Kitchen the Genovese started a war and won against the spillane gang.

Now even if the other westies wasn't a mafia fanboys they know that the possibility to made bussiness with italians for example the camion hikiacking and gave only the 10% to the italians was resonable same thing to be used as hired killers.

Second they haven't the men to start a war in 1978 there are (from a blackhandforum chart):

1)James "Jimmy" Coonan (1974-1988) First boss, Sentenced to 75 years in prison in 1988;
2)John "Jackie" Coonan Jr. (1974-1988) A soldier, brother of Jimmy Coonan, died of AIDS in 1988;
3)Bill Beatie (1974-1986) A soldier, formerly a member of the Spillane Gang, became an informant for the FBI in 1986;
4)Thomas "Tommy" Collins (1974-Pr) A soldier, sent to prison in 1988, released in 1997;
5)Francis "Mickey" Featherstone (1974-1986) Became Jimmy Coonan's second-in-command after Eddie Cummiskey was murdered in 1976, left the gang after becoming an informant for the FBI in 1986;
6) John "Johnny" Halo (1974-Pr);
7)Tommy Hess (1974-1982) A soldier, murdered in 1982;
8) Kevin Kelley;
9)Anton "Tony" Lucich (1974-1985) A soldier, passed away in 1985
10)James "Jimmy Mack" McElroy (1976-1993) A soldier, became an informant for the FBI in 1993;
11) Richard "Mugsy" Ritter (1974-Pr);
12)Richard "Richie" Ryan (1974-1986) A soldier, died of alcohol poisoning in 1986;

So if there are another men non listed are a small crew against almost 200 made men.