Concur with OldSub, snap up the M20 first. You don't want to give up the granny for hauling.

I am going to a SM465 with a gear splitter (got it cheaper than a gear vendors unit) to replace the crash box in my 39.

The SM465 doesn't buy you anything over the SM420 for all practical purposes. A backup light switch and a sychro first ain't worth the work.

I really like gear vendors stuff but, it is super expensive. The US Gear overdrive unit I am using will work and will work great for the price I paid used.

Since you have a vacuum rear end, you could add another lever to your floorboard in the form of an Advance Adapters Ranger Unit for less than $1500 giving you an overdrive (sort of) at a reasonable price. I like these units behind a 235 or a mild small block. I would leave it in overdrive for regular driving around town and drop it to direct for any necessary hauling. It moves your shifter back 7 inches and you will have to shorten your driveshaft. I mostly like these for price.

Or a Richmond Truck 6 speed with a 4:1 first (not short enough IMO) and an overdrive for about $3500.

Or a Dodge NV4500 using a Dodge to SM420 adapter plate would be great.