So that’s where Tom Calvert’s car has gone. Potter did a conversion on it.
IMHO, That Ford Aspire front end clip is certainly one of the aesthetically balanced front end mods I have seen. It compliments the cars styling nicely. This is the first I have seen/heard of Potters mods. It is the way Rootes would have had to build the car if they started stuffing 400+HP in the car. The Potter approach of boxing and using the trans cross brace as a structural member is a neat idea.
With respect to your car, the fact the someone else covered the original frame suggests that it is a patch job. The very weak link in the stock frame is where the cross frame, side frame by the rockers and the wheel arch frame all converge at the front of the rear leaf spring. Virtually every Tiger and many Alpines have cracks and frame separation at this juncture. For this reason it is a good idea to rebuild the frame. How you do it is your option. rebuild the stock or fabricate like Potter. Continue with leaf springs or go all the way and do coils. The latter requires a lot engineering work. If your are a proficient fabricator/engineer you can do it yourself. Other wise you have to pay some one. If paying someone who is a "professional", you will likely never recoup cost when selling the car. In other words, you should be willing to pay the price for uniqueness.
Good luck and have fun. Nothing is free.