Let's assume that free motor is a Windsor. Look at it like this.......that motor will be out of the car. This is where you save time tearing it down by yourself and cleaning up the block as much as you can. You're going to have to get the block machined in both cases for that 351W and that 302, regardless. Strokers have flooded the market and can be had relatively cheaper nowadays. Most of them cost the exact same regardless of what size motor you're building, but options will play a factor in price. You have to be patient and use your head. Shop around the net for the best prices on certain parts instead of just tossing money around in one place where they may charge more for a specific part than most other places. It's cheaper when you supply your own parts rather than have a shop supply everything. 1969 & 1970 have a 9.48 inch deck height. The 1971 through 1974 blocks have a 9.503 inch deck height. After that the 351W went to a 9.200 inch deck height. Piston selection is critical depending on the year block it is. I wouldn't re-use a stock crank, rods, pistons even if they were givien to me for free if you're going to make serious power. With the right intake components, good flowing heads, lots of tinkering & tuning I'd take a naturally aspirated 351W (stroked) versus a blown 302 based motor anyday. Once the motor is built all you would have to do is take a day to pull the old motor out and the new one drops right in and you could sell the old shortblock or part it out for some profit back.