“Unfortunately, not a very efficient dryer with the perforated drum back which allowed some of the heated air to travel behind the drum and not be forced through the drum”
Are you sure about that? The glass window on my 1963 Whirlpool dryer would get hot to the touch in operation on the high heat setting. Replaced all the cycling thermostats in October as a preventative measure since the originals weren’t that accurate and definitely were a little weak after many years of use, definitely sped up the drying time and is on par with a newer Whirlpool 29” dryer. Even dried a load of towels and observed the glow from the heating element in the upper left hand corner (the drum light was burned out so there wasn’t a UV light to give me a tan), the heating element stayed on for a good 40 minutes before cycling off vs with the old thermostats it would cycle off in about 10 minutes or less. I used to have to set towels between 4 and 5 but now is between the 3 and 4 mark.
I can see the incoming air bypassing the drum if you were to overload it but since I never overloaded my Whirlpool dryer, never had issues with long drying times.
Another thing too is you are pulling a large amount of air through a 4” circle where the heat inlet enters the rear bulkhead, the air won’t have much of a choice but to go through the drum, only way the incoming air will bypass the drum is if it’s overloaded or a restricted vent which will cause the incoming air to scoot behind the drum and out the exhaust since the air won’t be able to move as freely.