TOPIC: Setting up and fitting trailer bunks questions

I'd appreciate seasoned advice and links for guidance and recommendations for measuring and how to set up a tandem wheel boat trailer with center rollers and two bunks on each side to hold a vintage 71 Thunderbird Comanche I/O. I will be purchasing the boat from the water i.e. I've seen it on a storage yard trailer and it will be put into water for testing prior to purchase but does not have a trailer as it was kept in indoor rack storage. I have a line on a used, nice, new coupler, recent 15" tires, new LED lights, wide and long steel trailer that will allow her to sit low between the fenders and I can move the axels and cut some tube off the main tubes if need be. Thanks in advance for your assistance.