Hey, I got a USDM type R Gage cluster in my 94 dc4. The only thing that reads accurately are the RPM's, how do i go about fixing it or should i just toss it?

Here are the symptoms.
1. When I first start the car, the fuel gauge reads full, then after driving for about 30 seconds to a minute, it starts going down gradually. more so it goes to empty in about 10 seconds.

2. My speedometer reads 100mph when i'm doing 45 - 50. so when i'm doing 65 - 70 it's reading like 120 - 140, i mean it would be str8 if i was trying to spoof on video, but naawh i'll pass...

OK recent work that was done. i just threw another LS motor into her with a GSR tranny slapped on to her. so any help would be nice.