Exhaust opinion

Need to get out of my head and bounce it off of other people.My 360 currently has long tube headers,2.5 pipe to a flowmaster merged Y pipe Flowmaster Scavenger Series Y Collectors Y250300 to a full 3" single exhaust (tailpipe is late model F 150 take off and mandrel bent).
Engine is 8.75-1
Lunati hyd cam 220 @ .050
308 heads with 2.02/1.60 valves and bowl work
Port matched edelbrock performer intake
Proform 650 dp

904 tf with 2500 stall
8.25 with 3.90 limited slip

Sounds real fun till I tell you it is in my 87 Dodge D150 2wd.

Is my 3" single exhaust costing me much hp?