There are a few ways to add quite a bit power to your engine w/o removing it.
The easiest and cheapest would be to replace your oe rocker arms to cheby 250 rockers. It will cost about $50, and will give you higher valve lift and a bit more performance. Check over at fordsix.com forum in the FAQ section and do your research before starting.
Another upgrade would be to pull a set of EFI eshaust manifolds and install them with less restrictive muffs and cat.
If you still want to boost the hp more, then p/u a used 4v intake on ebay or craig's list, or at some swap meet, along with 4v vacuum secondary carb, rebuild the carb, and install. You will be amazed at the difference!!
Before you do any of it though, do your research and learn the ins and outs.