fuel psi dropping

cars been falling flat on its face at wot lately , bogging bad around town and popping out the exhaust when first started

i checked fuel pressure primes to 30, hold 30 at idle i pulled the vac line from fpr jumps to 40 if i rev car psi drops to 25 then stays at 28 psi after returning to idle
after car is off fuel pressure drops to 0 within 2mins
then i tried reving the car real slow and holding 2k rpm fuel pressure climbs to about 35 then drops right back off

fpr vac line has no gas in it no smell and was replaced less than 3k miles ago
fuel pump is original and has 94k miles on it

i tried two different gauges just to be sure it wasnt my gauge

i actually have a brand new fuel pump in the garage, how bad is it to change out. do you just drop the tank and replace or is there more to it. fuel pumps probably the only thing ive never done to these cars