how much motor

312 ci will be a big help. 10.5-1 comp.-ro will also be a help if you can run a good enough fuel to use the 10.5 - 1.  A good MSD ing. would also be a help.  MACK

My math says 4.2 MPH @ 2100 RPM's with a 13.6 X 38 (60 inches tall)rear tire in a WD-45 low gear. Which is 2.8 MPH @ 1400 RPM rated speed.

It's gonna be pretty fast with that stocks rearend so hold on lol!!!

Allis Puller you're running more cubes then mine. Internally little has been done to my engine. All I've done are a few air flow tricks. Now I'd be lying if I said I was completely satisfied with the performance but as far as money goes, the rear end swap gave me the best bang for my buck. In the near future is like to punch and stroke my engine but in the mean time I gotta run on a budget. I agree with the Dr...there are tons of small things you can do to get a lot more. Change one thing at a time and see how it affects the performance of the tractor. Don't do what I did at first (changing multiple things). Then you don't know what helps and hurts. In my opinion I would do the swap and then work on air movement. Engines are nothing more then big air pumps. It doesn't matter how big they are if they can't move the air in and out.

I thought you were going to put on the tires and try it??? Report back with the results.

How much bigger that stock is 56 allis motor

You will need a lower first gear or more hp.  MACK

56allis what has been done motor wise to your tractor

If you go from 16.9x26 to 13.6x38 you will need more than a 45D rearend.  MACK

Ive heard people have regreted changing rearends and i didnt wantto if i didnt have to and end up regretting it

Well designed drawbars and wheelie bars can accommodate the differing tire heights.

How fast are you pulling and what rear do you have now WD wd45 also what gear are you pulling in now

I don't know why you wouldn't at least TRY IT.

I knew a guy years ago, that pulled a Super 77 Oliver. He had 13.6 x 38 Firestone F-151 rear tires that were cut and only had about 3/8" inch tall lugs max. He had them on wide rims, like 15 or 16 inches wide I'd guess. The point is, I've seen him weight that tractor up to 9,000 pounds before and never kill the engine in 1st gear. His engine was relatively stock. You could have farmed with it. 60 HP max. His secret was the tires would finally spin out before the engine ever quit due to the wide rims fattening out the body of the tire to float on the dirt and the lugs being short, so they never could get a deep bite of the dirt. He won many pulls and beat a lot of higher HP tractors with the Oliver and it was all the tires, good weighting front/rear and a smart driver.

And how heavy do you weight it for with those tires??