Sorry but I have to dissagree. If I were fitting an EFi system to a kit car I'd go for a hotwire from a 3.9 or 4.2. These are a much more robust system than the 3.5 flapper, for which VAF sensors are hard to find, and will cope with small amounts of tuning on the 3.9, eg a sports pipe and K&N, so will cope with more on the 3.5. That said, a pair of EFi heads, 3.9 cam and sports pipe will see you with a good few extra horses in an engine that will still start first go.
You could pick up a ful hot-wire system for under £200, £150 of that being the ECU, so if you get that cheaper all the better. All the info for testing an EFi system at home with a multi meter can be found in the back of the Haynes manual for RRC.
If you wanmt to go crazy with it RPI will supply a Tornado chip for about £350.
This EFi system is what's in my mate's TVR 430HC and that's good for over 300BHP.