like joedoh advises, get the engine and trans in the truck where you want it so you have clearance for other systems and go from there building something. jack the unit up at your rake angle desired, block it up sturdy and level side to side, build a trans mount or, like joedoh says,use a stock one that you can just slide back for now but will hold the trans up, then mock up the other systems or at least think how they will have room to be installed-like steering column-exhaust-starter, then do a line up check to ensure the drive line is parallel to the truck frame so you don't get a shudder later, check the pinion angle and relate that to the engine/trans down angle so you also don't get a shudder later, bolt up some stock gm engine mounts to the block and make some cardboard patterns to connect them to the frame. the 4.3 v6 engine is simply a sbc with 2 cylinders lopped off so you could use a stock S10 4.3 mount and make patterns to get them connected to the frame. then take the patterns to a welding shop and have them made for you. probably no more than ordering a set that is full of slots and holes to fit everything from a v8 Kia swap to a rotary engine in a lawnmower, lol. try to use a non corrugated cardboard like poster board or some masonite type stuff that will mimic steel for non bending and that way not allow discrepancies. poster board for a general rough shape and then transfer that to the masonite for a firm final pattern. this will give the welding shop a better chance to finish up with a better product for you. mark the holes etc so they can be drilled for you and mark indexing on each piece so they know how part A fits with part B. if you really want to get after it you can pre-make all the parts and simply have them welded for you. an angle grinder with a metal cutting blade or zip disc will work wonders or a recip saw will get the general shape but be a little rougher around the edges.
personally I like the lennox metal max discs because they don't have a tendency to disintegrate while using them and they also don't get smaller as you use them.
https://www.lenoxtools.com/pages/lenox-metalmax.aspx
for recip blades ensure to get something made for steel with the number of teeth per inch needed for the thickness of metal you decide on. I also like to spray some soapy water on the blade while cutting to help with friction and heat that destroys the teeth quickly. being a retired firefighter I tend to opt for the fire and rescue blades as that is what I know works well.
post up some pics if you get a minute, so we can see what you have.