Apparently this is not going to be as easy as following someone else's example.
My frame cross section in the area behind the axle has a significantly wider lower flange (that is also tapered) than the upper flange. At the rear cross member the lower flanges are 29" apart, 29 3/4" apart at the cross angle location. Therefore the max tank width needs to be less than 29" to fit between the lower flanges.
The cross angle appears to be even with or slightly behind the rear differential. I believe the angle could be replaced/relocated forward slightly but as it sits the rear edge of the angle is 16 5/8" from the rear cross member. If it is relocated the existing front edge appears to be the furthest forward a tank could be and clear the differential rear cover. That dimension is more like 18 1/4" making the maximum fore/aft tank dimension approximately 18 1/4".
The Blazer tank measurements are okay for width (28 3/4") but the fore/aft dimension of 28 1/8" appears to be significnatly too long and would result in the tank overhanging the differential, possibly interferring as the suspension is loaded. The frame height is 4 1/4" so the 25 gallon Blazer tank at 10 3/4" high would hang 6 1/2" lower than the frame if mounted flush with the top of te frame.
Tanks Inc. shows a 1949-52 Chevy Fuel Tank 29-1/4" x 21-1/4" x 8-1/4" (16 Gallons) which seems too wide but may work depending on the overall width including a flange that would be either inside or below the frame channel. The length seems a little long but may be okay if the tank is moved as far back as possible with the rear flange either inside or below the rear cross member. https://www.tanksinc.com/index.cfm/...799/category_id=133/mode=prod/prd799.htm
the fuel filler location in the right rear coner looks good for a fender filler location but I am not sure how it will clear the frame rail unless the tank is mounted higher than the top of the frame (not sure about bed clearance). They also have an 18 gallon version that is 1" taller which may be okay if the top of the tank is above the frame rail for filler clearance.
Any insight from forum members would be apprecaited.