4g64 swap into my 86 G54 pickup

My 1986 Power Ram pickup runs fine, i rebuilt the motor during our Covid vacation, I put the 38 Weber carb on it and have Hooker super comp headers on it. Yes it's 4wd, 5 speed manual.

I found a pickup cab complete with motor, 5sp 4 wheel drive transmission and driveshafts. It has a 1991 4g64 motor. It was supposedly rebuilt before somebody chopped the bed and frame off to make themselves a trailer.

So please give me all opinions or experiences or advice you may have about me removing my perfectly good 2.6 liter and swapping in this unknown 1991 4G64 .

I'd like you to tell me I'll get better MPG and better horsepower and even better reliability with the fuel injection... but I don't want to assume it hope for that and later find the swap wasn't worth it.

Should I expect that the gearing of my 86 rear axle should match the gearing of the 91? Or should I just swap motors and keep my transmission in place??

Thanks again, guys, in advance. You've always been so helpful through the years and this feels like a big project I might regret?!??
Bill