You don't have a carburetor unless that's your plan to go to one. Not really any different that any other manifold swap. Just you don't have many or any easy choices for a different manifold. The hole in the back that the distributer goes thru is much larger than standard sb chevys. That's so you can angle in the distributer as it sits under the cowl. The dist is also a unique shorter piece to fit under the cowl. With machine work and possible welding to build up that hole so it can be machine, it can be done from what i've heard.
I have opened up the hole on a manifold for a 4bbl. It still fits, and the distributor works. If you are installing a carb, you may have to do more than just install the distributor, because the computer controls the timing. There is no other distributor that will fit under the cowl, because of the height. You probably have a 5.0L, and that is not worth swapping to a carb. Get a 350, and put AFR heads on it, then an aftermarket 4v tbi that can control the stock ignition system
If I can’t afford just for now a 350 where is a good start. Can u give me specific items to buy because I am not familiar with cars and I want to learn and diy my car projects. I kno I need an aftermarket intake manifold, what else can I do tho, I want new pipes should I do that myself or take it to somebody.?
There is a lot more you need to figure out than might strike you at first. The first thing I would do is drive it a while until you save enough money to do a major job. Dribbling a little bit at a time is really a waste. You can get a 350 from a pick-n-pull yard for about $400-500. You can get a freshen up kit for it on the cheap, and the only major expense for the block is having the crank turned. You can use a ball hone to prep the cylinders yourself if there is no big ridge at the top. If you want more HP, you can get a cam, and put new cam bearings in the block. Most cam kits come with new lifters. You currently have hydraulic roller lifters. Unless you are going for maximum HP, you can use the stock rockers. Heads are going to be your biggest expense. Do not skimp on the heads, that is the one thing that will make or break your engine. If you are going to up the HP, you must have the trans built to take the power. You should do the engine and trans at the same time, or do the trans first. Your cam will determine the stall RPM of your torque converter...more radical = higher RPM torque converter. If you are looking to go to a carb, consider the stand alone TBIs that can control your ignition system instead. You can get one for less than $1000, and it has its computer in the TBI. It will self tune. You will also get better gas mileage with the TBI.
To put headers on the 91, you must modify the trans crossmember to accept the left side pipe. You will probably find it easier to have turndowns in front of the rear axle. You can get a second muffler mount from the right side, and it will bolt up to the existing holes on left side. You will have a tight fit if you want to go around the tank on the left, and the pipe will require a heat shield along the tank.
The higher HP will also require a heavier duty radiator. There will be some mods to get it to fit, and you will want electric fans, instead of the mechanical one. Wiring for the fans is pretty straight forward. You will also lose the EGR, and the air injection. You should be able to adapt your throttle linkage, CC linkage, and trans linkage with a bracket. Most of the TBI units can not control and electronic transmission, so you will need the stock type cable operated trans.
If you put an engine in the car with less than 15 inches of vacuum, you need a hydroboost brake booster. The are a simple installation if you get the right unit (2006-7 GM 2500-3500 from the junk yard). You need a flat plate to mount it, and they come from early 90s astro vans, or you can get one on ebay for $25.
Depending on your final gear ratio, you may want to consider a gear change with a True Trac, or rebuildable limited slip differential. You probably have either a 2.73, or 3.23 ratio right now. If you have a 2.73, I would go to a 3.42, if you have a 3.23, you may be satisfied with that. I would still put a limited slip in it if you do not have one.
There are some specialized tools necessary as well. You will also need to do a lot of before and after measuring. This is pretty much the tip of he iceberg, and there are a lot of details that you will incur.