Glad to know the brain surgery went well. I'm guessing you aren't looking to go nuts with a build on the truck and are just looking to have some fun with it.
What you have is essentially the production offspring of the '89 Shelby Dakota.
As the other guys pointed out, the '91 has the throttle body injected 318. Depending on the emissions requirements in your area you might be tied to a stock combo there. I want to follow-up with Matt on a Megasquirt solution for these engines. I have a couple customers that could use that option on ther TBI Mopars.
Trans is either the A500 or A518. I'm 99% sure it's the A500. That's the overdrive version of the Mopar 904. Some guys still stick their noses up on them, but they can be built to take some power and live. A good shift kit will make it much more fun to drive. It has a steeper 1st & 2nd gear than the 518. The A500 matched to a 3.91 rear is a nice combo.
The rear should be the Mopar 8 1/4". In the Dak they were available with 3.21/3.55/3.91 gear options. Most Daks I've come across have 3.55's. 2WD ones seem to have 3.21 more often than the 4x4's. All were optional with a limited slip differential. Sure Grip in Mopar speak.
Ratios numerically as low as 2.21/2.45 were available in M-bodies, but require a different carrier. And they would suck the life out of the truck. 2.76/2.94/3.07 were available in other models, as were 4.10's. Some aftermarket gears in the 4.30/4.56 ranges too, but that's probably overkill.
Aftermarket limited slips are available too.
Early Daks can handle very well. There are some springs and sway bars out there for them. Some lowering spindles too. Ranger50 can give you the pros and cons to them.
The 6-lug wheel options are better than they used to be, but swapping rotors and rear axles out for the earlier 5-lug versions opens up more options.
On the engine, freeing up the exhaust can help. Later Magnum exhaust manifolds flow much better than the manifolds you have now. The best of the Magnum manifolds were the '92-93 manifolds. They had a larger outlet than the '94-up ones. Can be harder to find, but any 318 powered '93-94 Dak/Ram/Ram Van would have them.
The later manifolds still flow better than yours, so they are a good option. Shorty headers are available, as are long tube Hookers ($$$), but on a mild 318 I'd stick with Magnum manifolds. Getting a good Y-pipe bent up to a 3" single system would do wonders for the truck.
If engine swaps are a possibility, 360's in both LA and Magnum form will bolt in. Stroker kits are available for both to that them up to 390-408". A 340 will bolt in, but cores for a 340 are spendy. If you want to go nuts, a second generation Hemi (426 based) will fit.