47-55.1 Fuel tank advice

[51 3600]

Next project on my '51 is fuel system. Tankwise the goal is a rear mounted unit with filler neck up through left rear fender. Since my truck is a 3600 I don't have as much flexibility as the 3100s have. Space is more limited. Pic below shows space dimensions.

Note: bottom of frame rail is wider than top of frame rail. Bottom could be trimmed if necessary, just more work.
Attachment 2168018

I've narrowed my tank searches down to 2 Tanks Inc. possibilities a 53-CG or a 53-CPS. Originally used on 53-54 Chevy cars.

Attachment 2168023 Attachment 2168024

The steel 53-CG measures 29.25" x 21.25" x 8.25"
The poly 53-CPS measures 28.5" x 19.5" 8.5"
Using the poly tank I could get by without trimming the bottom frame rail. In either case I have to move the crossmember farther back but that's just unrivet, move and bolt or weld back in.

I know nothing about poly tanks so could use some advice. Pros & cons. How do you mount a poly tank?