Would a SELECT of the original tables using just the required columns help? or it wouldn't make any significant improvement?

Or maybe a view limiting the rows I need before I use the query on my SP?

SELECT * FROM(

SELECT

c.bilamtBILAMT_CT, --

c.cusnoCUSTNUM_CT, --

c.insnoINSNUN_CT, --

CONVERT(int, c.invno)INVNUM_CT, --

c.lnenoLNNUM_CT, --

c.procdePROCDE_CT, --

CASE WHEN ISDATE(c.stdos) = 1

THEN CONVERT(datetime,cast(c.stdos as char(8)),112)

ELSE '1/1/1800' ENDSTDOS_CT --

FROM AR_LOAD_DATA.dbo.clmtrk c WITH(NOLOCK)

UNION ALL

SELECT

h.bilamtBILAMT_CT,

h.cusnoCUSTNUM_CT,

h.insnoINSNUN_CT,

CONVERT(int, h.invno)INVNUM_CT,

h.lnenoLNNUM_CT,

h.procdePROCDE_CT,

ISNULL( CONVERT(datetime,cast(NULLIF(h.stdos, 0) as char(8)),112), '18000101')

STDOS_CT

FROM AR_LOAD_DATA.dbo.cltrkH h WITH(NOLOCK)) Claims

JOIN (SELECT 'A4310', 90,0 UNION ALL

SELECT 'A4311', 90,0 UNION ALL

SELECT 'J3490', 0,5 UNION ALL

SELECT 'L0631', 0,5 UNION ALL

SELECT 'L7900', 0,5)