I will try and be as detailed as possible.
-I have a 78 Bronco, 351 gears 35 inch tires
--I swapped in a Ford 300 I6 and a T-18 Tranny/Tcase from a 1980-81 ish Bronco.
---It has a single barrel carb, stock intake/exhaust
---Here is the issues, I have great low end torque, slow to moderate acceleration. When I shift from 2nd to 3rd, all is good. When I shift from 3rd to 4th,( RPMs around 2200-2500) my RPMs will drop around 1000-1200, and it will take FOREVER to gain speed. If I am going up a hill it will drop to 45 MPH and creep forever untill it can gain momentum and speed up. Once my MECHANICAL advance kicks in around 1600-ish, then I can gain speed again.
-----Things I have tried
-I installed a vacuume guage, no matter if I connect it to carb vac or man vac, it DOES NOT MAKE A DIFFERENCE. This may or may not be an issue. It changes wheather I have vaccume at idle, but once I am driving, if I shift and get back on it, the vaccume goes to zero.....
-I have advanced my distributor to max advance (total advance) and it RUNS GREAT!!!!! However, once I shut the truck off it will not start. I have it set now where cold start and hot start are both pretty easy.
---I have put a new distributor on
---I have changed the mechanical springs-set my advance stops
---all back to stock distributor now
Nothing fixed it. is there a distributor I can get that will be tunable for cold/hot starts, but kick mech advance in at 1200? is the vac advance ports faulty?
I realize 351 gears were not stock for a I6, and 78 is a bit heavier, but still.......
----Will a dual vac distributor help?

 

 

Tires are too large in diameter for the driveline specs and vehicle weight

 

Are you suggesting changing gear ratios? Up or Down? 3:42 or 3:73? and what would you expect as an outcome for each? I still need low end for deer hunting, and a good safe acceleration curve for highway.
I will also add when I swapped in a 460 and it was fine. but with the 300 I6.......not so much

 

It's impossible to know exactly what you have there for a motor but an all stock 1bbl 300 made as little as 88hp and 215tq while even the most smothered 460 made better than double that, what were you expecting?

 

Here's a couple of online reference charts/formulas which will guide you for selecting the proper gearset................................

Differential Gear Ratio (also known as Final gear). (4-wheeling-in-western-australia.com)

How To Select Proper Rear-End / Differential Gear Ratio (roadkillcustoms.com)

Differential Gear Ratio Calculator | West Coast Differentials

 

I will not say gutless wonder because I own a six pop too
They only had about 120 HP and 223 ft lbs of torque
That said it should still get out of it's own way if you change the gearing like one of the above posts mentioned
If you have 3.55 go 4.11 or possibly 4.30
Measure the circumference of both sets of tires and do the math
I think 4.11 will be close
Looks nice
Like the hood scoop

 

I wouldn't worry too much about your vacuum signal. Sounds like ported vacuum to me. Should have 0 at idle and 0 at wide open throttle. 1600 rpms sounds a little sluggish to me on the mechanical advance. I'd figure on around 800. But it all depends on how your distributor has been curved. Mine was setup so stiff it was basically locked out. I was looking for springs and couldn't find many choices. Ended up deciding my distributor was worn out and buying a NOS one from ebay. Between the two I ended up with a combination of springs I was happy with. Ended up using my original vacuum advance can as it was good and fit my needs. Took awhile pulling numbers and planning my curve before I got it dialed in how I wanted it. But, I think in your situation I'd start at square one. What shape is your carburetor in? If she doesn't scream like a scalded ape when you mash the go pedal, or act like one that's way over loaded, I'd assume a rebuild is in order. What's your base timing look like and manifold vacuum at idle? You said advancing the timing makes it run pretty good, but won't start. There's definitely some timing gains left on the table there and I'd say its the mechanical advance, but I'd definitely make sure she's running right first.

 

theese sites tell me 2.56 gears are best. Not sure if I will loose my low end torque though?

-the carb is about a year old. Still shiny and new.
- the distributor was bought new, but has a horrible curve. Finding springs is a treasure hunt. Is there an aftermarket distributor that i can set my mech advance around 800-1000?
— thank you for all the help. Gears are inevitable, but Id like to couple it with a good distributor.

 

Most everyone goes with a single wire hei type distributor. I'd gather data first. Map you timing curve and see what it looks like. Then order one of the available spring kits and see what combo you can get. Do you own a vacuum gauge?

 

What they are telling you is you currently have the equivalent of 2.56 diff gears given your tire diameter, etc.....

 

That is what I was thinking
What site and what info did he provide for them to recommend 2.56's?

 

I used these websites. I may have done it wrong, but 2.56 is what I am getting.it is telling me if I switch to 2.56 at 60 MPH and a manual trans i will be at 1475 RPM.

 

So at 10 miles an hour you will be at roughly 614rpm (with a guess at 2.50 ratio first gear)
And at 5MPH you will be at 314 RPM too low clutch burn type too low
I feel you are planning on going the wrong way on the gearing
You will loose what acceleration curve you had and the deer hunting gearing will be gone
Most cars are around 2500 RPM at freeway speed

 

Lets use your factory tire size(assume its, 235/75/15) and your rear gear 3.55, for you to keep the same git-up-an-go, with your 35" tires, your gearing would have to be 4.30, the least I would put under it, with the I6, is 3.90