Attaching Hyland Hitch to C/T

Your question is a little confusing.
If the hitch has bolt holes that line up, why wouldn't you just bolt it on, with the correct tensile strength bolts?
How was your old hitch fitted?
Is the Hyland hitch weldable?
Maybe a photo would help to understand better.

HT,
My understanding is by law that the hitches have to be bolted, you are no longer allowed to weld them.
Cheers
Baz

Howard
If you need the tow hitch to be bolted underneath for height purposes, it should be fine as long as the plate is welded properly and the bolts are of the correct grade.
As for the weakening of the RHS would depend on the thickness of the RHS.
If you dont have height issues i would just bolt it to the top plate and use the correct grade bolts, and all should be good.
One thing i have found with the hyland hitch is if you are using a genuine prado tow bar tongue ( cast one) you will have to get a hayman reese style tongue as it wider or have a stainless steel plate made ( same diameter as the socket ) to go under your tow ball above the tongue. The wider tongue is required to keep the socket that goes over the ball flat and straight. If its a prado style tongue or an aluminium riser tongue the socket will flop backwards / forwards around when travelling.
Hope this helps
Cheers

If in doubt, take the trailer to a trailer manufacturer or engineering/welder and get them to weld on a new base plate with the correct holes.