I am building a 54 3100 also. I am using an 18 gallon tank from a 68 camaro that will have through bed filling. Other options I have looked at that will fit include a 75-81 Camaro/TA tank (19 gal), 69 Mustnag (20 gal) and 70 Mustnag (22 gal). The Mustnag sending units run at a different ohmn rating than GM but can be made to work.
As to the grill the only thing you might have to modify is the top inside opening of the front fenders on the 54 are rounded and the 53< are squared off. I would keep the 54 fenders as they are and replace with earlier fenders. The earlier fenders are easy to find. I see them at Houston area swap meets regularly. I would suggest keeping the 54 grill as it makes it look unique from the ever popular 47-53.
I am in the Spring area if you ever need some help.
Welcome to the forum.
