There is a simple way to measure the weight of boat and trailer by using a tyre guage and ruler. First u need to know the tyre pressure in each tyre. Then slide 4 pieces of paper into the tyre where it contacts the pavement. So you end up with a piec...
I located several articles that address the isssue of welding dissimilar metals and have cited them below for you. I hope that they help you to resolve your question.
Steel is bought in a steel stockist. Usually found out of town, Look up yellow pages. It would not be possible for you to roll your own steel to different thicknesses.