That trailer is for a relatively heavy 455 Makocraft with 60 merc 4banger.
The trailbasher trailers are pretty well built. I would beef up the centre section and run with a solid keel support (Hardwood) but feel confident without much extra chassis work.
If you use 50x50 angle to mount the long springs to, you will find the spring loadings are then spread over a very long section of the outer chassis rails.
The channel trailcraft use is relatively light but should not flex overly badly.
As I mentioned before. If you dont intend to narrow up the track there is very little point in running larger than 14" LT tyres. If you move up to 16s and say 235/85s to match the fender you will have to upgrade the axle and hubs to take the load reliably. Once you head down that path it is ridiculous to not run same track width as tow vehicle.
Support the keel, change out the springs to something that can be classed as suspension (as opposed to trailer industry BS) a pair of dampers, and bolt her down solid and call it good.
S
