Unfortunately Steve your chassis was the last one to be built without the mods to allow easy fitment of the 24valve engines.
I think your biggest hurdle would be the rear cradle top crossrail which Neil mentioned above.
There is one out there but I lost touch with the guy who was building it and don't know what he had to do.
Last time I saw it the engine was in and sitting on the 12valve engine mounts I seem to remember, but there were issues to be resolved at the top end.
Corky rejigged his chassis and made new mounts I think.