Your shifters may be in a different spot.
I swapped an R151F in place of my W56 in my 88 and it pretty much bolted in. The plugs on the switches were different but, the switches swapped over from one trans to the other.
I used the same clutch and throw out bearing.
The shifters were the only thing that needed a little work. My W56 had a top shift T-case and the R151F had a forward shift. Worked out good because I combined the two to make a Toyota twin stick setup.
The R151F and the W56 are the same overall length so everything bolted right in. Not sure of the G54 dimensions though.
88 Pickup SAS'd, 3.4 V6, R151F Hybrid, Dual Ultimate, Triple sticks, 35's, 5.29's, HP front, V6 rear, Aussies, 63" Chevs, Warn 8274.