You really need to start with defining what this home-made tractor has to
do.

OK, you likely have the engine/trans from an old pickup. You say 2wd so I
assume you mean rear wheel drive and not front wheel drive. Building a
home-made tractor simply requires a rear diff ratio suitable for presumed
large rear tires.  If you used say 14" wheels and low aspect ratio tires,
the original rear diff ratio will be about 4.1:1. Put 16" wheels and high
aspect ratio tires and the engine will run lower rpm for the same road
speed, not that this would be the problem you envisage: the engine would
just be less efficient. Your cooling system would need to be adequate, and
have an electric fan. An oil cooler would be a good idea.

What you need is a diff ratio of at least 4.8:1. eg some sort of light truck
that has large wheels already. This will mean that you could start off in
2nd gear with no problem, leaving 1st for the heavy pulling.

About 25 years ago a mate of mine built a nifty AWD/4WD vehicle for his farm
in west of Sydney, Australia, before ATVs became available. Almost all the
parts came from the local wrecker. He used a 1275cc BMC Mini engine and
transmission mounted N-S. The original Mini diff became the center diff,
with unequal length prop shafts running to Datsun 1600 diffs running two
sets of Morris 1100 axles and uprights. He used 15" wheels as I recall. The
whole thing was built in a ladder frame chassis made from 2x4x1/8" tube,
with a roll bar and a 6'x4'6" box on the back.

The double reduction ratio with the two diffs was about 14.4:1, but with the
bigger wheels this became equivalent to about 9:1. In any case, you easily
could start off in 3rd with no load, 2nd for just about anything, and 1st
(flat out was about 10kmh) was only used to crawl over rocks and pull
stumps. After about 6 months he found the AWD centre diff action was
unnecesary on dirt, so he welded it up and made it 4WD.

SD