do you know what gears you have in the motorhome axle? what the width is like compared to the original axle? what size of brakes it has? what size of wheels it has? a lot of the motorhomes from that era has 16.5 wheel size, which are hard to find tires for. the good part is the 16" wheels also fit, you just gotta find set. yhr original apache wheels are likely a split rim, you don't wanna mess with those because the locking rings are known to blow off and take your life along with them as they fly across the yard.
the old 350 4 spd will do a lot of work for you but the top speed won't change until the rear axle gear ratio is changed. these trucks came with a v8 option so motor mounts are a breeze. they bolt onto the front of the V8 and the cross member below and then the rear is supported by the bell housing just like the inline 6 does. the difference is that the inline 6 bellhousing doesn't fit the V8 engine so you need to source a V8 engine bell housing that will mate up to your tansmission of choice.you may be able to get a few more miles per hour by getting a trans with overdrive or installing an overdrive unit behind the trans you have.
here are a fewlinks that show the engine mounts and the v8 bellhousing for reference. to check out overdrive units try gear vendors site
https://www.classicindustries.com/product/556038.html
https://www.ebay.com/itm/291979685639